I would like this very much as well. I would certainly buy one and recommend it to others!
Even better from my point of view would be a 49-key (or even a 61 key) version of the XKey Air without any left panel buttons. Preferably even the frame width would be minimal to allow the most natural playing across the two keyboards.
A quick-attachment method could be developed to hold the two boards together when you weren’t using them on a table?
A 49-key version without the left panel keys could be made about 29″ wide, which could still be used on your lap by itself, or combined with the 37-key unit to make an 86-key nearly full-size unit.