Is contesting with two rigs simultaneously something only for chestpounding bigguns or possible even without a fully automatted robot-station? During the last years I have tried a Single-Operator/Two Radio setup several times on the lowest possible level - with 5 W qrp and two multiband doublets. With this simple setup I got a feeling for the operating part of it without first having to invest in and deal with bandpass filters, interstation interference management and elaborate switching devices of all hard- and software kind.

What is it?
SO2R allows to listen on one radio while the other one is transmitting. With my 5 Watts output I had no relevant interstation interference even when the two provisional antennas were quite close together. Each doublet had its own matchbox, fixed wired with one rig - no antenna switching device yet. Such a device will be more important if you do not have equal (good) antennas. Then you should be able to swtich the better antenna quickly from one radio to another (it is easier to hear a fresh multiplier with the allband vertical than to work him with it because the beam can´t be used easily).

IC-718 and K2 each with an own matchbox for the two doublets. Having no elaborate scheme for the CW input I use two paddles.

No workout for the soldering iron
The only little building project necessary was a small box to switch the two audio streams in three different ways: Left audio for both ears, right audio for both ears (necessary when I want to concentrate completely on one signal) and third the REAL SO2R-position: right audio to right ear and left audio to left ear. What should be NEVER done - is simply soldering one audioline to the left channel pin of your headphones and the other rigs´s audioline to the right channel pin. Instant stereo receiving of two signals will destroy any fun with SO2R as well as efficiency.

Based on expert´s recommendation I added a variable resistor of 1k between the channels because our brain were better in channel separation with a bit of right channel´s audio in the left ear and vice versa - at least for me it works that way better than with full channel separation (try it and don´t argue with the grey software between the ears - do you argue with software in real life?)

Software
Talking about software - most if not all contest software is able to deal with SO2R and offer an overwhelming wealth of features - but also indicators for the operator what rig is active and what key will now do what. To give you an impression here a picture from the N1MM-manual. It takes some reading, understanding and some little experiments to get the basic feeling for it - but it is definitely not as hard as it may seem on the first look. But to get it intuitive you will need "some" hours of operating ;-)  For more elaborate setups the software packages also offer switching of audio, antennas etc., too.

How to do?
Now on with operating. The biggun´s most frequent use of SO2R is calling CQ with one radio and searching with the second radio for unworked stations, especially multipliers on the second radio on other bands (or checking condx on other bands or finding a free slot for changing the running band). For this a very widespread scheme is to switch audio by your software so that you will have the second radio´s audio mono in both ears while your first rig is transmitting (which should not need ear control). At the end of the CQ the software switches the first radio´s audio in again on one ear to let you listen for respondents and separates the second radio´s audio to the other ear. Depending on the situation you then may even switch to first radio´s audio completely in mono if copy is difficult.
The difficult moment comes when it´s time to call someone on the second radio without interrupting running too much on the first radio (either because of brutal stopping inmidst a transmission or having too long periods without transmission on the first radio which will allow other stations to occupy "your" frequency for calling CQ by themselves). The more difficulty you have on the second radio to get the wanted CQing-station with the first call and to pass the exchanges along without repeats the more difficult it will be to keep up with the necessary running-sequence on the first radio. One thing not so relevant for normal guys: Bigguns say that you can´t do much SO2R with a rate higher than 100 QSOs per hour.

SO2R for normal stations
More often we will have to do search&pounce on two radios simultaneously which is more difficult because you have lesser transmitting time in which you have do deal with only one received signal (I turn my transmit audio down but don´t switch it off because it is needed when infos have to be sent by paddle). But you have less pressure with interleaving qsos.

My favourite way to do it is using the "waiting time" when I don´t get a CQ-ing station with the first call. As soon as he responds to another caller I shift my attention to the other ear that has the S&P-audio from another band (or I then switch this channel in). While I look for other stations there, insert a call in the second log window and prepare to launch the call I simultaneously follow the qso on the first radio with "half" attention so that I can call in the right moment when the running station launches the next CQ. For me it is more to shift attention between both channels than to completely copy both signals simultaneously (which is yet possible only in few precious moments).

In this way you can either increase your rate by making one qso on one band and the next one on another band as here (remember: it´s only qrp):
20071020 1646 20    CW     LZ3FN        599 X06    599 173
20071020 1646 40    CW     UA4SU        599 X06    599 76
20071020 1648 20    CW     4L1QX        599 X06    599 56
20071020 1649 40    CW     YO5AJR       599 X06    599 28
20071020 1651 40    CW     OF3OJ        599 X06    599 119

- or by reducing the rate when you call a station on the second radio in vain while at the same time the running station on the first radio listens without a response (your transmitter is just busy with calling on the second radio - transmitting on two radios simultaneously is an absolute NO - even if technically possible in some setups).

You won´t transmit two signals at a time, will you?

To do or not to do - when?
So it is crucial to decide when it is time for SO2R and when not. You can go with your gut feeling when the rate on one band is good - which is no time to do intense SO2R other than checking condx on anempty band. Seemingly short openings to valuable regions are another not too good time to S&P elsewhere esprcially if you are qrp.This are also important moments for inband-SO2R (if you´ve tested properly BEFOREHAND that it won´t do any harm to your equipment - and then told your software to allow it)! If you don´t trust your gut feeling (which might come to strange conclusions say at 0400 morning local time) you better start to SO2R when the rate on the primary band drops below a certain value - say 70 with LP and good antennas or 25 with qrp and modest antennas. But that are rules of thumb to be exactly decided by every operator depending on his targets and how much the contest has progressed (and possible rates therefore diminished).

How long can you go?
The sound of SO2R adds an element of turmoil to your ears and brain. Even the best SO2R ops report increasing fatigue when doing intense SO2R up to points when it is no longer possible. You have to get a feeling when effiency drops too much because the internal QRM costs you more than it helps (a good sign is the error rate or missing letters when copying a call with having the second signal in the other ear). To reduce fatigue and improve efficiency it is very helpful to switch audio intensively. Usually I listen "mono" to one signal exclusively with both ears when I have to copy a real exchange like a serial number - which is also more fair to the running station not forcing him to send repeats. If fatigue increases I turn the audio on one rig down to only hear whether there is a station while S&Ping. A still helpful and less stressing use of SO2R is to instantly scan i.e. 15/10m for short openings (you CAN rely on clusterspots from elsewhere but you are better off with what is received at your station in realtime and before the hordes stumble in).

Getting better
A good way to have some training off air is feeding MorseRunner (simulating a running rig) in one ear and searching for stations on your real radio with the other ear. You can make even a little contest for yourself: multiply the number of MorseRunner qsos with the number of calls from the real radio (from different frequencies, no pile-up-stripping!) for 15 minutes.

How does it sound in reality?
Listen to K5ZD in WWDX with stereoheadphones and note i.e. how he switches audio.

When you´re hooked - and there is significant risk if you have read until this point you may be interested in the tech-stuff how to do it with LP or even HP (not only for hearing something on one band while transmitting on another but to let your rigs live longer) and to learn more about what hardware and software can add to automation. Look for fine descriptions by guys who are really into it:
SO2R-Overview from José Nunes, CT1BOH, regular WWDX-winner from CT3
SO2R-presentation by Stefan, DL1IAO, with precise schemes for qso-sequences and equipment
A typical hardware setup by Chris, G3SJJ

Thanks for helpful comments to Ed, N1UR and thanks in advance for YOUR comments, tips and experience

Woooow!!!!
You have read until this point. Thank you! That deserves a Refreshment´s refreshment. But be warned: Those are the most addictive 1:40 min I ever found on the net. So think well whether you really want to feel forced to hit the Again button every time you hear "Tack så mycket". Enjoy :-)))))