I'm second guessing myself - CTR on a deposit into a joint account. One person was conducting transaction. Do you complete 2 Part Is on the person conducting the transaction (2a and 2c)? I've never done it this way but am second guessing the instructions:

If more than one Item 2 option applies to a Part I person, a separate Part I section will be prepared on that person for each Item 2 option. For example, if the Part I person conducted a $5,000 deposit into their personal account and a separate $7,000 deposit into the account of another person/entity, there will be one Part I on that person reporting option 2a on the personal deposit with that amount and account number in Item 22 “Cash in amount…”. There will be a second Part I on that person reporting option 2b on the person/entity account transaction with that amount and account number in Item 22.

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.