Wooo!! OO-er, etc...
Congratulations :D
Yep, I have experience of twins. Mine are 18, but I haven't had any contact with them since they were 4 or 5. And my wife (now) is a specialised "twins" nanny. I haven't met her latest charges but I know the last lot quite well (my Fiesta Red strat is named after one of them).
Twins are, as you are probably hoping/fearing/expecting, both more rewarding and more of a nightmare than a singleton.
It's just like a singleton except it's twice most things and many more times more some other things. For example, if you have two kids already, you're about to introduce a further 8 person-to-person relationships into your home.
You're gonna have your work cut out, old chap :lol:. Make sure you both talk about it a lot, what your fears/expectations are, BEFORE they arrive - time will seem rather short the minute you get them home...
... anyway, that must have been a huge surprise. My first wife passed out when they told her :lol: (I'd already had some prior warning because I'd been watching the scan thinking "well, either the machine's f*cked, or that's more than one... or there's something terribly wrong...").
Congratulations again, to both of you :D