from the advanced routing howto, what you want is a Pseudo-bridge, this is explained:
Step by step, quoted from the Adv-Routing-HOWTO:
1. Assign an IP address to both interfaces, the 'left' and the 'right' one
2. Create routes so your machine knows which hosts reside on the left, and which on the right
3. Turn on proxy-ARP on both interfaces, echo 1 > /proc/sys/net/ipv4/conf/ethL/proxy_arp, echo 1 > /proc/sys/net/ipv4/conf/ethR/proxy_arp, where L and R stand for the numbers of your interfaces on the left and on the right side
For further information, please read the Net-HOWTO
for information on how to setup network interfaces.