I have YN685's and a 622tx ... just bought a Godox AD600BM with an X1c trigger. My question - how do I get the 622tx to operate the 685 flashes while stacked atop the x1 hotshoe? I have had several attempts but it is very flakey with the 622TX refusing to respond to commands when attached to the x1 (which is on my 5D3) Any help greatly appreciated
Someone on DPreview reports that stacking a YN-622-TX on top of an X1T works with the lights in manual mode. (link)
But the X1T's hotshoe is not a true TTL passthrough hotshoe. It is basically a slave in Group A, according to folks on POTN. Trying to use it to stack a non single-pin flash or trigger on top of the X1T may not work, because it will not have full hotshoe communication with the camera.
Or you could get a YN-622 transceiver and stack the X1T on top of that. The 622s have true passthrough hotshoes. They just don't have the nice LCD screen to display/control settings, so you'll probably have to use the camera's flash control menu to set the YN-685s.
Your best bet would probably be to get Godox X1R-C receivers for your YN-685s, or to trade them in for Godox TT685 or V860II speedlights.