How to Implement the back up Connection.

Dear Friends ,
Currently we have one radio link internet connection.We do have one VPN server for which right now the default gateway is Radio link service provder IP address. we want to have one other back up link for VPN server which we are planning to have DSL connection.As both DSL and radio link have output as ethernet port.We are planning to terminate our both the connection to Cisco 2620 router so in any case if one link goes down other take care with out any hindrance to our VPN server.Any idea what configuration we have to make in Router.