I was assuming that they would be on the same circuit..
I assume one will be on the garage CU, and other will be on house CU.
Use a relay connecting the coil across the lamp in the garage luminaire and the contact across the PIR sensor in the house luminaire.
Or even simpler connect both luminaires in parallel including the 'switched live' and both PIR's will operate both lamps. This method assumes the PIR is capable of switching the additional load, the instructions will give this info.
My sloppy sketch is using typical (eg B&Q/Wickes etc) low cost flood lampswhich have a 4way choccy in them.
The enclosure for the relay will need a warning label regarding more than one supply.