I received 2 CRP forms (M1PR) for the same apartment but under different dates for the same year. When I fill out the M1PR, do I just add line 3 from each form together?

Yes, add them together.

Renters: You must file Form M1PR and include all CRPs to claim a refund. Your refund will be denied or delayed if you do not include all CRPs when filing Form M1PR.