I am using two of them as insurance and they are both in the same room.
Although this will not cause an "ALL OFF" to be sent at 9:00 pm, it can cause problems. If both TM751s are in the same room, they are likely on the same circuit and same power phase.
I hate to contradict someone as helpful as
dave w but multiple TM751s interfering with each other is an urban legend. I did extensive testing a few years back without being able to generate a collision. Years and years ago, the TM751 (if that's what the old brown ones were called) waited for a rising zero crossing to transmit and this guaranteed that those on separate phases would always collide. That hasn't been the case for years. RF travels at the speed of light so all transceivers will
hear the signal within a few microseconds of each other and all now transmit to the powerline on the next zero crossing, irrespective of polarity, so its nearly impossible to generate a collision. You'll see more collisions from the supposedly polite transmitters which are guaranteed to collide if their numbers exceed the number of
random slots available. Also, many of the supposedly polite systems used TW523s which delay reports for 11 powerline cycles, long after the collision took place.
Several years ago there was a thread in comp.home.automation where I calculated how many miles apart two transceivers would need to be to see a significant difference in RF reception time. Of course, in reality X10 RF seldom makes it past 30-40 feet let alone several hundred miles.
What does cause collisions is multiple motion detectors that see the same motion a few seconds apart. People, pets, varmints, etc. do not move at lightspeed so this type of collision is common.