Question about Marmitek Subwoofer Anywhere 635

In principle, two receivers can be coupled to one transmitter and your idea could work under certain circumstances.
Connect the transmitter to a stereo pre-out.
On your active subwoofer you must be able to adjust the crossover separately. (Prevent booming at higher frequencies)
Your ceiling speakers are active speakers.
Whether this is a "clean" solution, however, is up to you :)

Greetings Dom