Supposedly, the rx3/tx3 is just wired to the same front port’s rx/tx but I can’t confim, just what Lacroix had said early on.
I have no idea what parallel pulling from the one port’s tx/rx pins would do either tbh. Mentally, I always thought “that seems like it should work” but then I saw the uSplit come out (device meant to split one uart port into 2 or more) and I had to assume that the answer was no.
Side note, @ricco is a super cool dude and is very knowledgeable and helpful about his projects.