You can combine them if you know the answer to one though surely? Are we talking about a difference between flipping a coin twice and flipping two coins at the same time?
I was following with HKFC, but you've torn my world asunder here. You're right, at least I think so. Girl/boy and boy/girl are of course identical if you don't care about which is the older sibling.
So you do start with:
1. Boy/boy
2. Boy/girl
3. Girl/boy
4. Girl/girl
And you have a roughly...