Re: One twin overeating
Hmm, I'm not sure I trust the advice you were given.
First of all, breastfed babies don't really overeat, because they're in charge of the feeding. The moment they start feeling full, their suckling slows down, and the flow from the breast slows down, too. This is entirely different from the way a bottle-fed baby eats: when a baby is bottle-fed, the adult is in charge of the feeding, and fluid continuously drips into the baby's mouth even after he is full and is only sucking for comfort, not nutrition.
Secondly, 2 oz of weight gain per day doesn't indicate overeating. Lots of breastfed babies gain very fast in the first few months (my DH, for example, was a breastfed baby who gained a pound per week), only to rapidly become lean once they become mobile and start expending their calories on activity instead of packing them on as fat. Rapid weight gain in breastfed babies during the early days is often about the baby's genes, not about something that the baby or mom is doing wrong.
So, what's the deal with the spit-up, gassiness, and pain? This resource may help you determine a cause. I would read the links on reflux and forceful letdown really carefully, since a lot of what you describe sounds like it could be due to either of those issues.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"