I want to layer my Xp-80 ( buying this Month)
sounds with the TR-Rack and be able to play
the TR sounds separately.

The layer part is solvable: Xp-80 audio outs
and TR audio out to my 8 channel mixer. Xp-80
midi out to TR midi in.

Question is can I do the following: Xp 80 Midi out to A50 Midi In ( has 2 In's, 1 regular and a 2nd In that ignores sysex from
originating keyboard), A50 Midi out to TR-Rack Midi in. XP-80 and TR audio out to mixer.
Will this let me play a layered sound playing
the Xp-80 plus let me play the TR-Rack from my A-50 ?

Or is this a dum approach and should I buy a Midi patch bay for the TR-Rack which (i think) will allow 2 Midi ins ?

It sounds like to me that you also need a roland A-70 controllerwihch has four midi outs and you can control everything, but get the A-70 with no sounds.