I suspect that it would be impossible to get a 2nd pregnant after a couple of weeks of the first. The baby produces hormones that stop ovulation. The 2nd one would have to be in before that window.
I also suspect that the birth of the 1st would induce the second. She likely didn’t leave labour, the 2nd just took longer, since it was, effectively, induced by the first.
Going back to the broader question, it’s likely that both were inside the mother at the same time. In theory they could be 9 months apart, though that would likely require significant medical effort, and so is effectively impossible.