Are you having problems with mount points in Typo3? Do you get errors like Illegal Mount Point found in rootline or The requested page didn't have a proper connection to the tree-root!, especially when you select the Substitute Mount Point (this page) with Mounted page option?
Assuming you did read the fine realurl manual and have done everything mentioned there, your errors probably originate from an outdated realurl pathcache. You can turn off realurl to confirm this.
If realurl is causing the errors, use your favorite (my)sql client and delete the contents of the tx_realurl_pathcache table:
delete from tx_realurl_pathcache;
This should allow you to have mount points with nice, SEO-friendly URLs across multiple domains.
Edit: If you don't want to mess with the database, see the fine manual for ways to disable and clear the cache.
Edit: If you have just performed an upgrade don't forget to run the database compare in the install tool.