I think your idea will or should work fine.... provided your own caveat of not energizing both pre/procs at the same time... and/or if there is some switching needed to be done on the Lexicon amp from xlr to rca.
Only issue I could possibly see is perhaps IF both xlr & rca are connected to the same inputs, one pair might take priority over the other, requiring you to unplug one pair entirely. Don't know for sure though... just maybe.
Everytime I've tried to connect my tube pre into a multi ch rig off a reciever, (for the front mains) I've introduced both lag time, and more noise. I finally just quit altogether trying to include the tube system into the HT... and that saves me another pair of ICs too.
So I just had - have, to switch speaker cables back and forth from one amp to the other. Neither did i ULTIMATELY CARE FOR THE AUDITORY DIFFS FROM ALL SS HT, to TUBE + SS, HT.
Just my 2 cents.