The movie plot, is as bubz said, those two are married and they're starting to get bored of each other, oblivious to the fact that they're both assassins, a secret they have kept from each other. Then they get told to kill each other. So, here's my prediction, the ending will be either:
a) they realise they love each other and don't kill each other, instead they fake their own deaths, then run away to an island in the Caribbean and make babies.
b) they realise they love each other and don't kill each other, instead they kill the people that have hired them, then run away to an island in the Caribbean and make babies.