Setting up second wan connection.

I live in a remote area in broadband terms.  The service we now enjoy is beamed from the next hill and acceptable but we have no fall back connection.  I do however have an ADSL copper connection which is available from a different ISP. This has a much lower bandwidth but would be available as a fallback option should the wireless link fail.  Is there a wiki which would get me started with setting this up please.  I am using an SG135 UTM. 

I searched for a guide but didn't find what I needed.

Grateful for any pointers please.

Budge