Oh, interesting. That was totally missing from the original article. I had assume that the twinning was because the egg had been fertilized by two sperm cells.
Rather, it seems that it's quite possible for any single-birth pregnancy to involve two sperm cells, and would result in a chimera. The fact that this one split into twins was a separate event.
The really simple model for an egg being fertilized by two sperm is that you'd get one fertilized zygote with three sets of chromosomes, fully triploid. Such a zygote would miscarry pretty quickly.
To get a less nonviable chimera, you need the egg to divide exactly once, such that one half gets one of the sperm and the other half gets the other one. If there is a mechanism that tends to ensure this happens, then it feels like that mechanism might also be related to the twinning (tripling, apparently) of the zygote under discussion here.
Rather, it seems that it's quite possible for any single-birth pregnancy to involve two sperm cells, and would result in a chimera. The fact that this one split into twins was a separate event.