Actually, now that I think about it, I could almost see us getting into a similar situation someday. DH has a nephew whose b-day is 2 days before our DS's. We may never have a "combined" party for the two kids, but, if we did, I would want to invite other people besides blood relatives (my kids' godparents) who wouldn't know the other child well. I wouldn't expect those people to provide gifts for both kids. Like I said before, DD's godparents give both of my kids gifts on each of their b-days, Christmas, & candy or stickers for other holidays. Generous people! We appreciate it, but don't expect it & certainly wouldn't discourage it, if they feel that's what they want to do.
A couple of times for Christmas gifts for my cousin's 2 kids, I got games that they could play together. One was an activity book that was a little advanced for the younger one, but pretty age-appropriate for the older one. Last year, I got them a tabletop basketball game. They don't mind getting something they can do together like that.
Sorry, got a little off the topic there! Just thinking as I'm typing & get a bit chatty!