So have the Nords and Bretons done anything else together apart from invade Sancre Tor?

The nords took a bunch of land from the bretons after the OB crisis. And....the nords killed a bunch of bretons in the early years of Mundus, because they thought they were mer.

there was an almost constant war between them over the boarder; for a long time half a city was in skyrim and the other half was in High Rock. read a summarry of Shadowkey, too.