This is not a build in the works, aside from my imagination. But I am wondering if anyone has ever seen someone split the spa and pool using glass..... but I am not talking about highly engineered thick glass to hold back the spa water.
The idea is that the glass only needs to hold back a couple of inches of water in the spa when the pump or pumps are running. I am sure the glass still needs to be an inch or so thick... but it's not holding back 1000 gallons and trying to be water tight.
In thinking about it, maybe the gap would need to be sealed in order to not require HUGE volumes of water to be pumped. But since water is on both sides, the glass isn't loaded like it would be if it were an aquarium.
Here is a sketch I made.