An easy and quick work-around, if you happen to own REAPER,
would be to use REAPER as a REWIRE Slave.
Route whatever outputs from tracks to appropriate ones in REAPER
through REWIRE, then do your parallel processing THERE.
Route the REAPER output to a Master Bus in SONAR.
SONAR's Audio Engine would still control the final output that way,
so no loss in audio quality, I would expect.
This is what I would do as I prefer SONAR's engine to REAPER's.
Mel