If there are multiple slates from a state that qualify for “safe harbor” protection, and if both the chambers agree to accept a particular slate, then that slate would count. Similarly, if both chambers agree to reject a particular slate, then that slate would not be counted.
However, if both chambers cannot agree to either accept or reject a slate, then the slate which was certified by the ‘executive of the state’ (generally considered the governor, but some interpretations would also consider Secretary of State to be an executive too – thus adding to the uncertainty) would be counted.
This determination of accepting or rejecting a slate of votes must be carried out in support of the State law and must respect the decision of the state’s election dispute resolution body (usually a court). The chambers may reject a slate if the electoral votes were ‘properly given’ – that is, there was no forgery, corruption, votes were cast by qualified electors who were voting on the correct day, electors voted for the correct candidate and complied with state laws that bind the electors.