hello,
just like ISDN, you must have floating statics pointing to your ADSL links for the IPSEC peer. Once your primary IPSEC tunnel is down, physical IP reachability to ur peer will be then provided using the higher metric route towards your ADSL line and the IPSEC negotiation will start towards your ADSL.
In this case, since the destination peer/interesting traffic is the same in both primary & backup, no additional configuration for IPSEC is required. Need to apply "cryto map xxx" on the ADSL interface.
Hope this helps !!