I tried it quite some time ago with two sessions and simulators. But, it wouldn't keep the sessions in sync. Might very much be my fault, though.
I think it would be a great feature to support multiple long exposure cameras. But I guess it could be tricky to fiddle with the exposure pattern, like 1800 s for Ha in Cam 1, but 6x300 s L on Cam 2 and 3x600s R,G,B on Cam 3. The master-sessions would have to wait for respective slave session(s) to complete. Certainly manageable, but not a quick/easy implementation I assume. If one messes up the exposure pattern it might also cause a horrible duty-cycle.
Anyway, INDI/Ekos/KStars has received so many great features over the last few years, I keep my hopes up that the multi-cam feature will find it's way into the code eventually.