A combined party?? Kinda defeats the purpose.
So let'e remove bachelor and bachelorette from the title and just say you've been nominated to throw them a gongrats party.
You should call some local night clubs and ask them if they have a sectioned off area you can rent.
Open bar or pay as you go?? You can try to collect cash from everyone invited to help with the bill then the bar will run up a tab you are responsible for at the end...
A bar is the only thing that comes to mind for me... music, maybe pool and darts, and everybody is happy.