Yeah, I think you would have to figure out some way to tell the router to allow both of them to access the internet, but not allow the two DECAs to access each other. I don't know enough about routers to know how to do it, but I would think there would be ways to accomplish it.
Otherwise I wonder if it wouldn't send some kind of a red flag for DirecTV to investigate if it saw receivers from two different accounts on one DECA network. Otherwise you could have people sharing receivers to get access to programming they don't subscribe to, kind of like account stacking.