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!