[thaumasiotes]

Given that this appears likely to cause the death of one or both twins (on the assumption, not true here, that there's no such thing as prophylactic surgery to remove precancerous gonads), it doesn't seem like the rarity really requires an explanation. Contra what the article says, these twins are not in good health.

If you're asking "what is the mechanism that makes sure this doesn't happen?", as opposed to "why doesn't this happen more?", that's a better question and I don't know the answer.