During the PRM approach, the monitor controller (who works at the TRACON) operates on the tower frequency. You do not communicate with the "final monitor" unless there is a transgression into the NTZ.
After being vectored onto at least a 15 mile final and as soon as the approach controller verifies you are tracking the LOC inbound, you are handed off to the tower frequency. At that time you're supposed to switch to tower, as well as begin monitoring "the monitor" frequency. All that is, is just an identical copy of the tower frequency, so what you're hearing is normal. A final monitor controller is able to override the local controller on the same frequency, but is not able to override another aircraft speaking on the same frequency, thus the reason for having two separate frequencies.