In practice, what will happen is that Mike Pence will announce that he has received two envelopes from Pennsylvania (if in fact he does), and that as presiding officer he will rule that the votes signed by the PA legislature will be counted and those signed by the Governor will be discarded.
There will be immediate objections. The vote counting act allows rulings from the chair to be appealed to the floor, so Pence’s ruling will be voted on by the Joint Session, whose decision is final.
There will be immediate objections. The vote counting act allows rulings from the chair to be appealed to the floor, so Pence’s ruling will be voted on by the Joint Session, whose decision is final.
From my reading of the EV counting Act, it's the two separate chambers, not the joint session, that vote regarding competing slates. So you'll have the House voting for the governor/SoS's Biden slate and the Senate voting for the state legislature's Trump slate (assuming Republicans win at least one Georgia Senate seat). Given that standoff, the Act states Congress shall choose the governor/SoS's slate. But that conflicts with the Constitution that says state legislatures determine the manner of selecting electors. Even worse, there's no consensus regarding the meaning of the term "majority" of electoral votes. Still sounds like a brewing Constitutional crisis to me.
In some respects, it's similar to 2000, when the SCOTUS needed to step in to stop a potential catastrophe. The obstinate Florida SC intentionally ignored the SCOTUS and wanted to keep doing recounts and make the mess far worse. So the SCOTUS finally decided to intervene to avoid a never-ending disaster in Florida.
But 2020 is far more complex. First, there's a Democrat Party Cabal international conspiracy that has perpetrated massive election fraud, thus seeking to totally destroy U.S. elections and therefore the very foundation of the country. And second, it involves at least six states, possibly many more (or even all 50 states?). So sometime in the near future, I expect the SCOTUS to once again weigh in, just as it did in 2000. I could be wrong (given so many RINOs) but that's my WAG.