That's an interesting question. My Pass Aleph monoamps have also XLR- and RCA-outputs - but no switch. I mailed Pass, if it is possible to connect two preamps at the same time.
That's their answer:
It is possible, in theory. The challenge you will face is to not have inputs coming from two different preamps simultaneously. Most preamps are designed to be left on all the time, and they will produce a low level signal even if not in use, depending on their circuitry. This will cause "confusion" in the power amp (receiving signals from two different inputs). Also, if you were to drive two signals by accident into the amp you could cause problems.
So, if you experiment, be very careful to only drive one signal at a time into the amplifiers!
Peter Perkins, North American sales, Pass Labs and Volksamp
I am not sure, if I should trie it.
Graz, home town of the Terminator...