I don't have twins and mine aren't as close as your kids (only 15 months apart) but I use an LL Bean diaper bag that I love (
Diaper Bag: Shoulder Bags and Packs at L.L.Bean). I've bought other bags because I wanted something new but I've always tossed them aside and pick up my trusty LL Bean bag. I've used it every day for 2 1/2 years and it still looks brand new.
You might also look at Land's End because they might have something to suit your needs.
Good luck.