I grew up with an older brother whose birthday was exactly 5 years before mine. The dual birthday party was OK when we were little, but by the time I was in elementary school, all I wanted was to have my own special day, and not have to share it. This got even worse when I eventually married a woman whose birthday was ALSO on the same day, but that's besides the point.
What I really want to communicate is that each kid should get their own special day, don't treat it like a parenting two-fer. They're individuals and should be treated as such.
As for the invitations, yeah say that it's a joint birthday party, but that it's not expected of them to bring gifts for both of your children.