Probably related to maged.eg's post: How can I paint across two texture sets? e.g. If I would want a long stroke of paint from the hood to the back.
It's been a while since I used substance software and I'm hoping someone could help here or link a good tutorial for that.
according to the rules we're allowed to adjust the UV maps, if you change it to something like this . . .

you can fit the UVs of Body A and B on to a single material at a scale of 75%
you'll lose some quality, but at least be it will enable you paint & project across the whole body. It depends on if you're willing to make the trade off in texture resolution. screenshot of the difference between the 2 below.
Original UVs at 4k . .

UVs at a scale of 75% (so 3k i guess . . is there was such a thing)
