They are not entirely enemies. Its just that they have different masters. Anyhow, I think Rajak was there because there's a war brewing against Lukedonia and the Union.
They are not enemies to begin with. It was Raskreia who sent Rajak to bring Rai and there he got caught up with Franken. This time also, he was ordered to check on the situation after getting Seira's report. In fact, I think both Franken and Rajak respect each other...remember when Rajak scarred himself to compensate for Rael's sneak attack?