Hi L Medlin,
So there are 8 stereo FX engines in the SQ, which you can use pretty much however you want.
When you send multiple channels to an FX unit (Mix -> Return), they are summed. So as you point out, when you then route the FX return somewhere, it will contain the affected sound of everything you have sent to that FX unit.
In your situation, if you need to have separate reverbs for monitoring, then you would use separate FX units for each vocalist.
Hope this explains it, but for more clarity you could study the processing block diagram in the technical datasheet and trace out the signal path.
Cheers!
Keith.