I had a small separation (diastasis recti) following the birth of my twins. Although I did the sit up test at my 12 week check and got the all-clear from my GP (airily said "oh, that looks okay") I wasn't convinced and spoke to my pilates instructor who said that I had about two fingers worth of separation - not much in the grand scheme of things but wasn't going to go back by itself and would just make my back problems worse (that didn't actually feel possible at the time...). Mine was just below my belly button which is why this area has always looked lousy since birth and my beautiful, sleek naval is now a hideous wide blob (sob!).
I avoided ALl crunch and oblique abdominals as these tend to make things worse and 'fix' the abdominals in place, making closure of the gap impossible. There are specific exercises you can do (google the Tupler Technique) but you have to be very patient and committed to see results, which let's face it, is pretty hard when you're looking after twins. However, it is possible. Pilates also really helps as some of these exercises can help close the separation. However, it should always be done with a trained instructor rather than running through a DVD as they will be able to tailor it to your specific needs and avoid any of the aggressive exercises, that would just exacerbate the problem.
The problem with a twin pregnancy is the muscles have to move very quickly and the uterus grows very large so the rectus abdominals can 'tear', causing the disastasis. Normally after pregnancy the muscles slowly come together of their own accord, but after a diastasis they can' tand need help.
I had physio on my scar, post section, as it had masses of scar tissue due to complications, and the physio I saw specialised in closing abdominal diastasis - she saw heaps of mothers of twins and had excellent results. She could usually shrink the gap significantly through physio and specific abdominal movements. A two fingers separation is seen as 'okay' and she was getting resuls from 8 fingers separation down to about 3 or 2.
Hope this helps. There are things that can be done and although some separations are very severe and require surgery, others can be massively improved through specific exercises.
Kx