Sync'ing the information would probably be the best method, however as a quick solution even putting up a static page to say "we're having trouble, come back soon" is much better than a 404 or non resolving page.

Also - rsync is great for sync'ing up sites (static pages). From a Database end of things, requires a little extra tweaking. The only issue at that point would be after you've gone from "A" to "B", when "A" comes back online, the information must be sync'ed back and can create a nightmare.

Load balanced machines take care of this - and it's even possible to setup load balanacing through multiple data centers.

Personally I'd setup a server with a database only. Then use two other sites/servers to access the database - thus information is always available. Then you just have to make sure that the mysql server doesn't go offline