I do not think the 120 minutes would be devided by the number of members of Senate or the House. That would mean each congressman would get only 16.6 seconds, making meaningful discussion impossible. That is not the way it works.
3 U.S. Code § 17. Same; limit of debate in each HouseWhen the two Houses separate to decide upon an objection that may have been made to the counting of any electoral vote or votes from any State, or other question arising in the matter, each Senator and Representative may speak to such objection or question five minutes, and not more than once; but after such debate shall have lasted two hours it shall be the duty of the presiding officer of each House to put the main question without further debate.
https://www.law.cornell.edu/uscode/3 U.S. Code § 17. limit of debate in each HouseThis brief report from the Congressional Research Service may answer many of your questions.
Counting Electoral Votes: An Overview of Procedures at the Joint Session, Including Objections by Members of Congress
Updated December 8, 2020https://fas.org/Counting Electoral Votes: An Overview of Procedures at the Joint Session
