The legislation that governs how electoral votes will be counted in Congress is the Electoral Count Act. It is long and complex, but to boil it down to the parts relevant to your question:
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The Vice President (who chairs the joint session) is required to open the certificate of attainment and any document purporting to be a certificate of attainment for each state. The question has been asked whether this means that I can mail in a “certificate of attainment” for Lyndon LaRouche scribbled on a cocktail napkin and have it opened at the joint session. The VP could probably ignore such an obvious crank, but a document sent in by the Trump electors would likely be opened.
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When the joint session receives more than one purported set of electors from a state, there are three scenarios outlined in the statute:
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If only one of the returns was certified in compliance with state laws for appointing electors by the safe harbor date, then it must be counted as the true return. For every state but Wisconsin, their electors were certified under state law by the safe harbor date.
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If more than two returns meet the safe harbor requirements (because of dueling state governments or state government officials), then neither may be counted unless both chambers agree one is the correct return. This does not apply to any state this cycle.
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If no returns meet the safe harbor requirements, one return may be counted if both chambers agree it is the correct return. But the statute further says in this instance that if the chambers disagree on which is the correct return, then the electors certified by the Governor of the state will be counted. Wisconsin Governor Tony Evers has certified Biden’s electors.
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So, even if the renegade Trump electors are able to have their certificates opened by the joint session, the statute dictates that the Biden electors must be the ones counted. One Senator and one Representative may jointly object to the Biden electors, but those electors will be counted unless both chambers vote to reject them. As noted, House members will vote singly and not as state delegations.