I know the PIX Firewall Version 6.3 or higher allows the use of the DHCP server on any interface. Previous versions only allowed the use of the DHCP server on the inside interface.
I have not tried it on two interfaces at the same time but I don't see why you couldn't.
You could also think of using the DHCP Relay Agent feature. Acting as a DHCP relay agent, when the PIX firewall receives a request from a host on an interface, it forwards the request to a user-configured DHCP server on another interface.
Hope this helps,
Curt