Dual Ethernet NIC on the same network

My question is similar, but not equal, to this one: http://www.debianhelp.org/node/14944
I have a network were the server is a Debian Etch machine (headless server), this machine serves as the gateway, DNS and DHCP server.
It has 2 main interfaces, the internet connection (let's say ppp0) and the LAN connection (let's say eth0).

One of the machines on the LAN has 2 ethernet interfaces (eth0 and eth1), which I'd like to use as efficiently as possible.
Unfortunately, bonding them into a single interface is not possible due to the switch limitations.
In the past, before the etch server moved in, there was a hardware router, the pc was running Ubuntu, and by default the 2 interfaces would both get an IP from the router and the routing table looked like this:
Destination **** Gateway ****** Genmask ******* Use Iface
10.0.0.0 ------ * ------------255.255.255.0 -- eth0
10.0.0.0 ------ * ------------255.255.255.0 -- eth1
default ------- 10.0.0.50 ----0.0.0.0 -------- eth1
This resulted in every connection to the internet going over eth1 (default gateway), and every (outgoing) connection to the LAN using the other, eth0 (as it is listed first)

After replacing the hardware router with the debian etch machine, this setup no longer worked, there were frequent (every 5 seconds) disconnects (it reconnected immediately, but downloads stalled because of it and all other programs reported a disconnected session), so I had to remove 1 of the cable from one of the interfaces.
Now the pc in the LAN runs debian Lenny (desktop), and I had another go with the dual NIC, now it doesn't disconnect anymore, but it seems to disable 1 of the interfaces (no IP assigned), probably holding it as a fail-over?

How do I get that old behavior back? Considering the following:
- Both interfaces need to be configured from the DHCP (preferably)
- 1 of the interfaces needs a static IP, which is decided by the DHCP server, based on it's mac-address. (Working perfectly)
- the one with the static IP needs to be used for the internet connections (there are incoming internet connection routed to it by the gateway)

Can anyone help me with this? Or if their is a better solution for this.
I think the ideal situation is a bond, but the switch doesn't support that.

Best regards,
Steven