Re: Issues going from one-sided feedings back to both sides?
I think that what you're seeing sounds very normal. Your baby has been working really hard to fill his tummy on just one breast. Now he's using 2 breasts and he probably doesn't yet know how to adjust his nursing style to compensate for using 2 breasts at a feeding, and getting full faster. Result: a very full tummy and greater likelihood of spit-up!
The great thing about spit-up is that when a baby is otherwise happy, healthy, and growing, it's a laundry problem, not a health problem. So I personally would be totally okay with spit-up as the price of faster feedings. Just lay in the burp cloths and extra onesies, and remember to swab out those neck folds once a day so spit-up doesn't fester in there. Your baby will eventually figure out how to feed in a way that is less likely to result in an overfull tummy.
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!"