In order to setup your ASAs to failover over you need to determine which one you want as the primary unit and secondary unit. On the primary you need to setup failover using the code:
failover lan unit primary
failover lan interface Failover GigabitEthernet0/3 (You can chance the GE0/ whatever you want. Both units need to use the same failover port and have a crossover cable between them.)
failover link Failover GigabitEthernet0/3 (Keep the same as the above port)
failover interface ip Failover 192.168.254.1 255.255.255.252 standby 192.168.254.2 (Create a small network so the two boxes can talk. This will replicate the config from the primary to the secondary.)
Then to determine what interfaces goes down to failover use this command:
Include all interfaces/vlans you want to monitor and if one fails it will failover to the secondary unit.
Once you have the failover setup, all the changes you add to your primary ASA will also be added to your secondary.