IIRC, legally WUMG and WAVM are two separate licenses that are "sharing time" on two facilities that are mutually exclusive to each other for 24/7 operation. In this case, the two facilities are actually the same transmitter and antenna array, AFAIK, but legally each licensee is responsible for their own facility. In real terms, that means when it's WUMG's time, the legal responsibility falls to UMass Boston. When it's WAVM's time, it's Maynard H.S.'s responsibility.
In even more real terms, I'm sure UMB set it up so the same remote control can be accessed by the appropriate personnel at both stations...probably with no time restrictions built-in to the hardware but undoubtedly it's understood that when WUMG is in active license, the students are WAVM are not to fiddle with things. At least, not without clearing it with UMB first.
IIRC, technically there is a dividing line of "official time" for WUMG to be in the driver's seat, but it's also set up in the real world that whenever there's no Maynard students, then WAVM will also rebroadcast WUMB as if WUMG were still in the driver's seat. (i.e. an LMA)