It's not too common, but common enough. When your seed formed, it grew 2 embryos instead of just one. A di-embryotic dicotyledon is the term I believe. There should be nothing wrong with either sprout. I wouldn't try to separate them until they get a little more mature.
Whether it was caused by the "stress" of feminization or not, I don't know.
It would be interesting if they turn out to be the same pheno (identical twin) or not (fraternal twin).
Hick might be able to impart some wisdom on this interesting subject.