I wouldn't worry over much about the stalls at this point. I would work on getting your horses healthy and use to people. Bring little treats out when you go and see them feed mom if babies won't come to you and slowly start offering the treats to the babies. If the babies haven't been handled very much there is no rush to get a halter on, I would guess they were not imprinted at birth so everything is going to be new for them.
It is entirely possible that they are not twins and the mother took on a second foal from another horse. We once had two mares give birth 4 days apart, the foal from one abandoned his mother and stuck with the other mare like glue. I guess he just didn't like her. She ended up bringing him up and nursing him along with her filly. After a while she wouldn't even let the other mare near him. I am not sure how common this is but I do know from experience it could happen. (It was the first time we ever had mares give birth so close together)
And of course congratulations are in order. Best of luck with your new baby!