I suspect it's a case of 2 same house code transceivers messing each others signal up. Like
Brian, I don't know if these are polite transceivers, but some how I doubt there was ever a need for them to be.
Having 2 set to the same house code during an actual break-in could conceivable cause no security lights nor external power horns to be activated.
This is an interesting scenario, so here are a couple thoughts that hopeful spark some more ideas from others as well:
1) Use 2 different house codes. This of course would require 2 different sets of security lights and external power horns. You could still use your remotes as common to each system, but there would still be a signal crash when you try to turn on the light(s). Also, if during a breach, a sensor from each system gets triggered, signal crashes will start again.
2) Completely isolate one system:
- Use 2 separate house codes
- have 1st system set up normal with the phone line, all security lights and any external power horns; as well as a spare DS10A not connected to any window or door (more on this below)
- plug an X10 filter into the wall and plug a power bar into the filter; connect the 2nd system into the power bar as well as a Universal Module
- set the universal modules house code to the same as the 2nd system, but make sure the unit code is different than the PS561's
- wire that spare DS10A to the Universal Module so that when the Universal Module is addressed it will trigger the DS10A
This should isolate the 2nd system from the main house wiring and allow it to trip the 1st system if any of it's sensors get breached.
Others thoughts, ideas, corrections or flaws to these suggestions are encouraged.