I think it may be both. I can't see one letting the other fall without going too.
If one falls, the other goes nuts and kills everyone else. Twins have that strange bond. Magic twins must really have a weird bond.
And Rowling seems to do everything for a purpose. Something's up with Ginny, the first girl in generations and I think something will be up with the twins. (Granted, they aren't the only twins in the series.)