i read somewhere that you can float a plywood subfloor. I have 2 layers of 3/4 plywood to install plus 3/4 solid oak trying to build up level with gym floor. If you do float the plywood do you fasten the plywood together or let them float also?

Here's how I approach that type of install:
1: lay bottom layer of plywood the long way of the space, leaving one inch gap along walls and 1/4 inch gap between the sheets.
2: lay top layer of plywood diagonally over bottom layer, same spacing, and attach to bottom layer with adhesive and staples.

This creates a room sized sheet of 1 1/2 inch plywood to affiz your hardwood to.