I think there is a misunderstanding: SunSDR2-software does not allow a real SO2R/duplex yet. That is supposed to come in V3. Simultaneous transmit is not ready yet.
I have experienced with dxlog in SO2R-mode with 2 keyboards to control both rx, each one as a separate radio. COM-connection is done for both RX like described in UMA, it makes both rx compatible as separate radios in dxlog. Remember that COM is a pair, so COM5 in the sdr will be COM6 in dxlog. CW/keying is done via DTR/RTS. When I want to transmit on the 2nd radio/rx I swap the radios with CTRL-S (or shortcut F10), grab the multi and swap back to the 1st radio/rx for CQ. That's it. All radio-control is done via E-Coder.
Still it is actually more like SO2V but without dunestar, extra so2r-controller, microham-boxes etc. I am sure that the real SO2R will come, but filters are needed.
I love SunSDR with its integrated skimmer, that makes it very easy. SDC is a great software, but I use it only used for skimmer and its / cluster-integration for dxlog. COM-spider is not needed for this constellation.
If you habe any questions, mail me.