Depends what you do. Casual talk is probably the best in the beginning. I recently did the Love after Breakup scenario, and the situation was the same. Both sims were, if my memory serves, Deeply wounded. I tried for days to patch things up between them, but found no matter what I did they worked against me. Romantic interactions were out of the question! Had to settle for new love interests eventually.
You could take them on a vacation, maybe find a place with very few other sims. Less distractions. If they want social contact with anyone, they only have eachother. It could work, or it could go horribly wrong :D