I had to break up a situation where the Sarranids and the Swadians were literally always ganging up on the Rhodoks. Made for interesting gameplay the first time around since it made being a Rhodok warlord fun, especially since those are probably the two scariest countries in the game if they get going, but in the end it was just too predictable. Killed replay outright. And for me it took Swadia and especially the Sarranids right off the list of potential vassals, since I prefer signing up as a vassal to an underdog.
So what I did, was make the Sarranids a little more eager to sieze the city of Dhirim, which is always a bit of a hot potato anyway. So now it's a bit more free flowing, and yes, the Rhodoks and Swadians will frequently ally, after they've gotten the initial war out of their systems. And just as frequently they'll live their lives not bothering each other. This is most likely to happen after the Swads have lost some ground in another front and need to close one potential trouble source and shore up their position to keep either the Sarranids or the Nords at bay somehow.