Hmm, usually the ones i install have backup power in each unit.. However, if you think about how you wire interconnected smokies, you use the same gauge wire for the low voltage interconnecting signal
So, I would run another wire of the same gauge and voltage rating, probably a blue or yellow wire, and use it for the backup power, does the backup require both a + & - or just one wire?
I see no code violation as long as all conductors can handle the highest voltage that will be present and conduit fill is considered