Back in 1960 there were two slates of electors that were selected because it was Hawaii's first voting year as a State and it was done under court supervision while the court finalized supervising the recount. But that situation was resolved BEFORE Congress met to count the votes so there was only a SINGLE slate of electors from Hawaii.
The votes for Nixon had already been certified, but Kennedy had filed a challenge. That challenge was still pending on the day the electors had to meet to cast their votes (Dec 19). Two sets of electors cast votes, and after the recount was completed (Dec 28) Kennedy was declared the winner and the Governor re-certified the electoral votes with Kennedy's electors.
In Georgia, virtually the same thing happened. Trump had a pending challenge which the court said on Dec 9 it would consider "in the normal course". Kemp had said on Dec 7 that Trump's challenge was still viable. Trump filed a petition with the Georgia Supreme Court on Dec 11 seeking timely relief, and was denied.
They had to meet and cast votes by Dec. 14 which is the deadline in the ECA (3 USC 7), or Trump's electors would not be counted if he ultimately prevailed. So they did what Hawaii did in 1960.
The section of code you linked for 3 USC 15 was amended in 2022. The ECA in 2020 clearly envisioned the situation of multiple slates of electors. (apologize for the text, it's "as printed", and I still cut it off. link after)
"If more than one return or paper purporting to be a return from a State shall have been received by the President of the Senate, those votes, and those only, shall be counted which shall have been regularly given by the electors who are shown by the determination mentioned in section 5 of this title to have been appointed, if the determination in said section provided for shall have been made, or by such successors or substitutes, in case of a vacancy in the board of electors so ascertained, as have been appointed to fill such vacancy in the mode provided by the laws of the State; but in case there shall arise the question which of two or more of such State authorities determining what electors have been appointed, as mentioned in section 5 of this title, is the lawful tribunal of such State, the votes regularly given of those electors, and those only, of such State shall be counted whose title as electors the two Houses, acting separately, shall concurrently decide is supported by the decision of such State so authorized by its law; and in such case of more than one return or paper purporting to be a return from a State, if there shall have been no such determination of the question in the State aforesaid, then those votes, and those only, shall be counted which the two Houses shall concurrently decide were cast by lawful electors appointed in accordance with the laws of the State"
I will also direct you to this CNN piece from 2020, in which Larry Lessig recommended that the Biden campaign do the very same thing in Pennsylvania.
abcnews.go.com
www.heritage.org
