^bump
very similar setup here.
i make music on my macbook, game on my desktop. I suspect it will work if i use the mac as the mouse / keyboard server, but that means introducing lag into the gaming setup. I am going to test it anyway. maybe with a wired connection it wont be so bad but it means the mac will have to be docked in order for me to use the PC at all (or start fumbling with cables i suppose)
-Ill mention here that sidecar does work with synergy. If i want to extend my mac display with sidecar I can move the synergy mouse (controlled by the PC) to the ipad no problem.