The RTC cannot be a master, but more Arduinos can be. However, there are problems because it is more complicated situation and the libraries not solve it now. Easier would be to have only one master to serve others include to putting the time. The RTC is able to generate precise signal e.g. 1Hz to sync all.
I don't know the RTC board you have but as I see, it has DS1307 and ATtiny chip. I don't know the purpose of ATtiny but just briefly thinking, this chip could be a master also.