Seems difficult. Not bonded, so lower thermal conductivity. You can’t press something against the glass if you want to keep it flat. Hot air won’t have any more thermal conductivity than the free air above the plate, so thermodynamically you will have trouble maintaining temperature without making the air in the box under the plates quite hot. Are you thinking of taping down top-side thermistors in use?
Personally, if I were doing this and required to use glass rather than cast aluminum for the bed surface, and cast aluminum were not available, I think I would bond to heaters (with thin aluminum sheet as a heat spreader if necessary for the heater type), with connectors to disconnect power and thermistor for washing off the top surface, and use high-quality electric connections so they didn’t catch fire.
Thinking wayyy outside the box and going beyond what I’ve heard of anyone doing before, since it sounds like you’ve already thought of that, have you thought about a water bath in that frame with the water touching the plates when it is at target temperature? You could use tape on the bottom sides of the seams between the plates to keep water from getting to the print, perhaps. Probably you would want to seal the top down to the box too. Then a pressure relief off to the side; a filler tube that goes above the surface level, combined with a dump valve to let water out before removing the plates? It’s a crazy idea and I doubt it could work, but maybe it triggers someone else’s ideas that are better?