The configuration for each interface needs to have something like this:
primary unit
int e0/0
nameif inside
ip add 10.10.10.1 255.255.255.0 standby 10.10.10.2
sec 100
When the primary and secondary unit sync they will have the same configuration in each interface.
You will get the following on the secondary unit
int e0/0
nameif inside
ip add 10.10.10.1 255.255.255.0 standby 10.10.10.2
sec 100
The important thing is to have the two units with the right failover configuration, on unit will be the active unit and the other the secondary unit.
I hope this helps.
