If you are putting down 1 1/2 xps, you certainly would not need 3/4 ply on top of that.. If you thinking anchoring things to it, there are ways to cut down on the amount of plywood you are using.. Lumber has gotten extremely expensive..
I am currently building cabinets out of XPS, I have been using heavy construction Gorilla silicone.. the stuff is awesome.. in my opinion better than liquid nails(I tried both).
if you attach the XPS to the floor I would certainly use the gorilla glue adhesive. If you are able to preplan for anchor points, the gorilla glue adhesive will bond xps and wood very well together..
3/4 in plywood though on the floor simply would be over kill the xps will not crush all you would really need is 1/2 heck 1/4 inch would be fine for a surface to walk on..
I would thinking about just adding some 1x2's to give you some anchor points, then put 1/4 in ply down for the floor.. you will save on weight, and give a couple more inches to play with height wise..
just my honest opinion