I get it OP, I started a similar thread when I found out I was expecting my second DS.
Of course I was delighted to find out that my baby was healthy and I feel so lucky to have had an easy pregnancy. This pregnancy has been quite different to my previous one so I assumed I was expecting a girl. I realise now that all pregnancies are different, regardless of sex, but I'd be lying if I said I wasn't a little disappointed. This will be my last DC, so no daughters for me.
Do you know what, I am now completely over that and am so excited to be a mum of 2 boys. I'm glad I found out the sex because it's given me time to deal with these feelings in my own time, instead of dealing with them when I've just given birth, am overwhelmed by hormones and feel like I've been hit by a train. I'll bet that in a little while you'll be just as excited about having one of each instead of 2 DDs and like others have said, who knows how your children's personalities will pan out as they grow up.
For what it's worth, my DS is a complete whirlwind, he is noisy and the house always looks like a hurricane has just been through, BUT he's also a proper mummy's boy, really snuggly and totally loving. My best friend's daughter on the other hand hates cuddles and throws tantrums if you so much as look at her wrong! Who knows whether it's got anything to do with their sex, but there are so many things to love about boys.
Also, I'm one of 4, 2 sisters and 1 brother. I am by far closest with my brother, always have been, so there is no reason why your children won't necessarily be best of friends.
Congratulations 