Thanks for explaining. It’s hard to get my head around the idea that this isn’t a settled point but of course we are in uncharted waters.
OK, I found a clue. I found this account of what happened in 2005 when Democrats objected to the inclusion of Ohio’s electoral votes.
https://www.cnn.com/2005/ALLPOLITICS/01/06/electoral.vote.1718/
“If one member of each body of Congress objects, congressional rules require that lawmakers return to their chambers to vote on the merits.
“A simple majority vote in each chamber would overturn the challenge — something that should be easily achieved in the GOP-controlled Congress.”
https://www.cnn.com/2005/ALLPOLITICS/01/06/electoral.vote/
“Alleging widespread ‘irregularities’ on Election Day, a group of Democrats in Congress objected earlier Thursday to the counting of Ohio’s 20 electoral votes.
“The challenge was defeated 267-31 by the House and 74-1 by the Senate, clearing the way for the joint session to count the votes from the remaining states.”
So in 2005 the vote on whether or not to count electoral votes that were objected to was done by simple majorities in both the House and the Senate. I would expect the same procedure to be followed in 2021.