secondary IP address on an interface - how?

[hws_admin]

I've a PIX-515E running 7.2.2 and I'm trying to create a secondary IP address on the outside interface. It's like having eth0 and eth0:0 on Linux.

If I run "ip address ..." in config-if mode, it just replaces the existing address.

I'm not sure what else to try.

[hws_admin]

Oh, I see, it's a subinterface.

http://www.cisco.com/en/US/docs/security/asa/asa70/configuration/guide/intrface.html#wp1044006