Almost... What you're saying is only true for fraternal or dizygotic twins. It's possible for a fertilized egg to divide in a way that it separates and both cells grow into twins, these are called monozygotic twins. Now, usually monozygotic twins are usually of the same gender but, there can be mutations. And even in those, the chance of both being fertile is rarer, because of the chance that the mutations usually are X0 leading to Turner's syndrome in the female child.

https://en.wikipedia.org/wiki/Twin (Read the unusual twinning part of the article.)