If they are twins suggests that these two materia have been put in a linked slot.

There are just too many possibilities, especially as it's a specific two materia. I was thinking MP Turbo because it increases MP cost but increases damage, but have thought too much about it and am now more confused.

EDIT: But if I look at the hint about opposites, I'm going to take a wild guess and say: Revive & Destruct. When they're mastered, they are perfect opposites (one causing instant death, one causing full life). Otherwise I'm going to have a long think about it.