Only one can operate. Logically its the one at the leading end of the train, however, there are instances where the engineer will operate from another unit. These are very limited circumstances, a conductor must be at the lead end, and speed is restricted.
Operationally however, any one cab stand can control an MU train, (with limitations) if you try to key in a second, the train will dump.
There's a safety interlock that WILL NOT allow a second set of cab controls to function.
The Cab in use must be completely "made safe" before using another cab in the set.