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Keeping mirrors isn't something for shared hosting, since it involves server administration (see posts above). What you need to do is find a way to synchronise your databases and cache/, templates_c/ and uploads/ folders (and the best way to do this requires server admin access). I'm no expert on this, and it's not something XOOPS can help you with as it simply lies beyond the scope of the XOOPS CMS.
As for making sure a mirror is used when the primary system is down, that is something you can do with XOOPS. Assuming the database is unreachable (if the whole server is down then the script can't detect anything, so it's useless then), there is an error message that tells you this (Due to an error this page can't be displayed, etc. error in databasefactory.php line xxx). Find where that message is generated, and change it to redirect to the mirror server. That should forward any traffic when this eror occurs.
There's not much more XOOPS can do, because as I said before, it's a server load issue handled at the server level or even at the network level. Definately not at the CMS level.
Herko
