DHCP and PPPOE simultaneously - how?

I require a local address (via DHCP, in order to access the local network; games, files, etc) as well as a PPPOE connection (for internet). I have Tiger 10.4 and connect via the airport. However, it seems like a PPPOE connection and a DHCP lease are mutually exclusive; when PPPOE is enabled, the DHCP option disappears.

I've already done some searching, but the only thing I've found is this from the apple support site, which doesn't work:
Mac OS X 10.4: How to use a PPPoE connection while maintaining Bonjour for local network devices
It requests that you make a copy of the PPPOE connection and then enable DHCP. I don't see how that's meant to help though; one option is still mutually exclusive of the other.

Help appreciated.

Can you give us a bit more detail about your hardware setup. On the face of it I'd say just get a cheap router and let that handle DHCP. But could do with more info.

The airport connects to a wireless AP, which is connected by cable to my college's server, which runs DHCP for internal connections and also facilitates PPPOE (an individual connection for each room) for external connections. I really don't know much else than that.

On both windows and linux I am able to set DHCP and PPPOE up as separate connections/interfaces even though they are accessed through the same physical nic.