If an official check is made jointly to two payees and is deposited into an account held by only one of the payees, can it then be considered an "other check" by virtue of the check being deposited into an account held by someone other than the payee of the check?

I do not understand the question, but can offer an unequivocal observation that any check payable to two parties must either be 1) endorsed by both parties or 2) deposited to an account owned by both parties to which both parties have equal access.

Getting back to your question, under Regulation CC (which is what I believe you are asking about), a cashier's, teller's or certified check payable to more than one party is considered a next-day availability item if deposited to the account of a payee of the check. It doesn't have to be deposited to an account in the name of all of the payees [229.10(c)(1)(v)(A)] provided it's deposited with an employee of the bank with a special deposit ticket (if the bank requires the use of a special ticket to attribute next-day status to the check).

That said, I agree with Ken if the check is payable to multiple parties not in the alternative (use of AND or & between the payee names). But if the payees are named in the alternative (use of OR or a virgule or forward slash (/) or simply stacking the names up one over the other with no connecting text), any one of the payees has full rights in the check and an indorsement of any one of them is sufficient.