We often have a dual birthday party, what we do is a bit different. When we have parties, we encourage our children to "give back" on their birthday, so we ask our guests not to bring gifts - but to bring something we can donate. We pick a charity (local animal shelter, food bank, etc) and look up their want list, and ask our friends to bring those items. We beleive a birthday party is a great opportunity to teach our children how to be generous :)
If that is not something your family wants to do, you could still accept gifts, and just not open them during the party. You could allow your children to open their gifts separately, that way they won't notice if one child gets more than the other.
Have fun!