ASA, FWSM? Routed, Transparent?
On FWSM, we use Active-Active in both routed and transparent. We use link-monitoring to determine which interfaces initiate a failover.
It will not work with 2 FWSMs in the same chassis, because the VLAN-interface only goes down when there are no other active ports. With 2 FWSMs in that VLAN, the interface does not go down.
When in different chassis, make sure only one port on the switch is in that VLAN (only int vlan x *OR* switchport access VLAN x, but not both), otherwise the link will not fail on the FWSM.
Hope this helps,
Marcel