If the person is making a deposit to their own account is still just 2a even if there is a joint owner.
See the note to CTR FAQ #23 on deposits to a joint account.
Note: If Jane Smith did not conduct a deposit, but John Smith deposited $12,000 into the joint account, there would only be two Part Is. One Part I would be for John Smith as 2a "Person conducting transaction on own behalf" and entering $12,000 in Item 21 and the account number affected. The other Part I would be for Jane smith, checking 2c "Person on whose behalf transaction was conducted," entering $12,000 in Item 21, and providing the account number affected.
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