What about expanding on the idea floated below of one gender having a sleepover Friday night and the other Saturday night. Let's say the boys have overnight friends on Friday, and the girls just sleep in their own room, no friends. On Friday night, the girls make themselves scarce and the boys do what they want (DVD choices, themed cake, decor, etc.), followed by breakfast the next day. Then the girls' friends arrive around noon, and ALL the kids do something together (make their own pizzas for lunch, watch another DVD if all can agree, etc.). (Alternatively you can go out to one of the suggested things below, like mini golf or bumper cars, if it's not prohibitive in terms of finances or carpooling.) Then the boy guests go home at 3 PM and your sons make themselves scarce at home. If you go out to a venue, then the boy guests get picked up there by their parents (having been taken there by you from your house), and the girl guests get dropped off there and get taken back to your house by you.
Then the girls' party begins, with whatever activity they want, and their cake/gifts, and then their sleepover. In the morning, the girl guests go home after breakfast, and you and your husband collapse.
Your sons and daughters could choose their own respective themes for decor and theme (cake, paper plates etc.) but have them agree on a color scheme for things like balloons if they want that - you aren't doing 2 sets of the more expensive things or dealing with picking up a 2nd set of balloons mid-weekend.
It's a ton of logistics and you absolutely have to have iron clad RSVPs from the other kids.
I think if the kids' birthday comes mid-week, you could split it into 2 parties, especially after the kids get old enough that they aren't sharing friends or interests. But for now, if they do, great. It's a ton of work but it's not much worse with one marathon weekend vs. 2 separate ones.