This is already done. Check out the Hubduino project.
This could be done through a Parent/Child Relationship I assume but someone with more experience with that would have to chime. The one question I had after reading your question was why did you want them to be in the same device? Add the parent/child devices actually would leave you with 3 devices in the end instead of 2. So, actually, it would be more complicated. What are you trying to accomplish by having them in the same "meta device" as you call it?
