The best thing to do is ask them. Ask the question to them together, and give them the option to answer you privately. That way there is no pressure for one of them to answer "the way you want them to". But explain to them that if they have two parties they may need to be smaller than if they had one together.
I understand where you are coming from. My younger sister's birthday is the day before mine. So we always had combined parties. I remember as a child never feeling like mine was that special because I always had to share. However, now I enjoy it because my sister and I are very close, and I think it's great!
I hope I was able to help you even a little bit! Good luck! If you have anymore questions or I can be of anymore help, please feel free to message me!
M.