Dr. Pepper-
There are many possible answers. The most obvious one is triplets, as given by Mod 80. But they could also be quads or quints, etc.
Other answers include the possibility that both are test tube babies who were implanted into different surrogates. This would mean they both have the same parents, but since they did not share a womb they are not twins. The coinciding birth dates is a quaint coincidence.
Alternatively, they could both be clones. Their shared “parents” would be the donor cells they were generated from.
Or, you could be looking at individuals from parallel universes. One or both may have made the transition through a different dimension (though obviously not the 5th, or I would have noticed) to our world. Same parentage and birth dates are obvious in this case, but they couldn’t be called twins.
As I said, there are a myraid of possibilities.