If the UVs are stacked, Substance will overlay the AO results of both, making it appear black. Try unstacking mirrored faces by offsetting them -1 outside the UV 0,1 frame. Note: any lowpoly faces outside the UV frame will not project its highpoly reference to the bake.
You just set the weld threshold property before opening the window. case modKey() of( #ctrl_shift_alt: ($.weldThreshold = 0.1; $.PopupDialog #WeldSelected))
Lokii288, for you: https://15104268797498972201.googlegroups.com/attach/72af0a3433963dea/Untitled-2.jpg?part=0.1&view=1&vt=ANaJVrE48oPj_xeu4xCrEINI0PS0unfGZ0tf1cCpA8uyAb5GBO4sy5Bx5ztPCp9scgv2hD8453ioQOg3m29weKIjrxOfzu4xlANejNDUrJfznY4PqwOYrIc