I think I may have been under-feeding them. They eat so fast that I was cutting them off after a certain amount, gradually stepped up from when they started. Well last night when the big one should have been sleeping he just kept crying and crying. When I'd pick him up and snuggle him he'd settle down and start sucking on my hand. When I'd put him back in the box he'd start yelling again. So I thought ok maybe he's still hungry. So I warmed up a little more formula and he ate 3 more ml. Eep! So after that I've been making them eat more slowly and burping them at intervals throughout but letting them stop when they're ready. And they have almost doubled when they were eating. So maybe that would have caused some of the minimal pooping. We'll see how it goes from here.
I'll weigh them tonight after I feed them.