While you can link multiple cameras to one monitor, unfortunately each camera can only be registered to one monitor at any given time.
In other words, with the MBP36, it is not possible to have two monitors linked to the same camera at the same time.
The MBP160 is a system that has 2 parent units linked to one camera. You may wish to consider moving to a WiFi video monitor system which allows a camera to be viewed securely on multiple smart phones/devices via an APP.
Blink1, Focuss66 and MBP85Connect are among Motorola Monitor systems with this capability.