There are several problems with this:
1) If both the TM751 and the CM15A pick up commands from an RF remote or a motion sensor, it is possible the macro that they trigger may be run twice.
2) The TM751 is "impolite," meaning it does not wait for the line to be clear to transmit. If the CM15A is transmitting a PLC command, and the TM751 gets something at the same time, it could step on the CM15A's transmission, and both signals could get lost.
I suggest trying one of the CM15A antenna modifications first. The "18.5-inch reflector mod" is the simplest, takes almost no time, and does not void the warranty. All you need is a piece of stiff wire (a straightened coat hanger works fine), about 18.5 inches long, and some tape. You simply tape the wire to the CM15A's antenna (no electrical connection is made - the wire "catches" the RF signals, and "bounces" them to the CM15A's antenna).
If that doesn't help, then you might want to look for an RR501 instead of the TM751. It is "polite," and will wait until the line is clear before transmitting.