I have two customer with dual ISP and I configure the asa5505 with the wan in failover (wan1: outside, wan2: backup)
It seems to me that everything is working well: when to outside go down, the backup go up.
The only thing that I did not well understand is the public IP address that I can use to reach the ASA remotely to make troubleshooting. For example, I installed the last asa, I see that the default route is on the outside interface (S* 0.0.0.0 0.0.0.0 [1/0] via 83.x.x.1, outside),
but I cannot reach the ASA in ssh on this IP (83.x.x.2) but only on the IP of the backup connection.