How can I power a load from two separate circuit breakers?
Use a simpler relay

A better plan would be to use a UL-listed SPST relay with a 120V coil driven from the motion sensor, with its contacts in parallel with the lightswitch.  (A Functional Devices RIBU1C or RIBU1S will do the trick, provided your lighting load does not exceed 600W.)  That way, all your lighting is powered from one breaker, and you don't have to do a bunch of rewiring.

Of course, this requires that your motion sensor power itself through the neutral, or be replaced with one that does, instead of "cheating" and trickling power through the load.