I haven't had multiples, but I had a gigantic belly (people thought I was due when I was 6.5-7 months, and by the time I really WAS due, people looked at me like I was crazy when I assured them there was only one! lol), and I had significant diastasis. I also have a "jowels of the dog" belly, as Kate Gosselin calls it!
Anyways, I did crunches with my hands on my sides pulling in towards the middle. I didn't start until probably 2 months PP, but I'm sure I could have started sooner.
Can I also say how STRONG my abs are now?? I guess from separating, then having to repair themselves, they got SUPER strong. I can go from lying flat to sitting up with only the slightest bit of effort now! (Too bad nobody will ever see my "abs of steel" through the huge flap of extra skin! lol)
As for the "trophy belly", I haven't found anything to get rid of it yet, but I have found that belly dancing brought it down a bit. Only thing is, I find that as I lose weight (I still have almost 45 lbs. of the almost 90 lbs. I gained while pregnant), my body shrinks but the skin doesn't, so it actually becomes more noticeable.
Personally, I think I SHOULD have gotten another baby for all the changes my body went through!