Not sure this can be done - I have one ASA 5510 with 4 interfaces. I want to be able to firewall all internal vlans that would connect through a single inside ASA interface (Inside) but pick-off one of the vlan's traffic so it exited a second ASA external interface (Outside2) going to a second provider - the first provider would handle traffic through Outside1 for all vlans but the one picked off. There would be no route sharing between the two providers so respective traffic would return on the same ASA interface it exited. I don't see how to default gateway for that one vlan out that second Outside2 interface.