Multiple masters for i2c device
You are correct. Multimaster must be handled by you. i2c Multi-Master description. The Main thing for a proper multimaster system is that it understands arbitration logic, and can tell when the system is busy. The first is more difficult than the latter. To tell when the system is busy, each multi-master must be able to check for a start condition, and wait for a stop condition before trying to use the bus. Arbitration logic is for situations where both masters might try to take over the bus at the same time.
But there are other ways to do it, outside of the i2c standard. You can use an extra pin each of the masters, and make a simple busy signal standard.

Check if Busy pin is On.
If Off, set a status variable to On and Busy pin to On, then talk to the rtc. When done, set variable and pin to Off.
If On, check if status variable is On (Did &quot;I&quot; set status variable to On?). If no, loop until busy pin is Off.