Identical? 

The main YA article states that Billy and Tommy are "identical" twins. Even though I understand they they look remarkebly similar I doubt they are actually Identical. Identical means the same DNA, same DNA should mean identical mutant powers. Unless I have a misunderstanding of how the X-Gene works, that would mean that they would have to be fraternal twins. It would make sense, becuase Wanda and Pietro are fraternal twins. There are instances where fraternal twins favor each other simply because they are siblings, which could be the case for Wiccan and Speed.