a) You cant have 2 SBS servers in a domain.
What you can do this - take a look at these products
http://www.doubletake.com/english/products/sbs-edition/Pages/default.aspxhttp://www.heroware.com/This product replicates everything to cloud, so you have high-availability written all over it. If exchange goes down - user's can be asked to go to the cloud and still get all their mails.
i havent used this product myself.
Hope this helps.