I just poll digital pins between Arduino waystations, if you will. With nothing else going on on say, Arduino Nano B that switches a 12V Auto LED with an MOS module, you'll not miss the signal of a digitalRead(lightBPin). Personally, I like the KISS rule.
I'm uber amateur and maybe it's not the most elegant way, I don't know. But I pulled enough hair out in some builds trying to get multi-devices working on one Arduino until it occurred to me that Arduinos don't cost very much and are very well suited to doing one thing, very well.