Usually a good regular sized backpack works great as a diaper bag for twins and can hold plenty of diapers and supplies for your twin babies.
However some other good diaper bags for twins would be.
Bably Baby Large Capacity Diaper Bag Backpack, Bag Nation Diaper Bag Backpack, Jujube Be Prepared Diaper Bag, Kate Spade Watson Lane Stevie Baby Bag, Diaper Dude Messenger Diaper Bag and the DadGear Backpack Diaper Bag.
Those are all some good diaper bags for twins with plenty of room for the diapers and supplies needed for twins.
Also another good diaper bag for twins is the Skip Hop Duo Double Diaper Bag.