The ulr rewriter don't consider host if you are using a normal licence.
It actully dont use it as a key for the cache.
If you have enabled the enterprise version and have 2 sites in the web.config you should be ok
it's an enterprise version yes. I figured it out during the day. two different pages on two different languages had the same 'page name in web address'. it was a bit hard to spot, but very easy to solve when it was found. I'm thinking it might be good to implement checks to avoid this in the future. Maybe newer version EPi already has this? (running 5.1.422.256)