Getting 2 Birman kittens, what genders go together best?

They are indeed lovely cats. Lucky you! As to gender, I don't think it matters, as long as they are littermates. I've had brother-sister littermates, and right now I have two dear brother -- in both cases they seem to be each others' best-friend-for-life. So you will have some happy times with your new friends, and seeing how much you care about their welfare, I believe they will have a happy life with you.