I assume you are talking about ethernet bridging so that you have one computer in front on the other on the internet. So that you can have one computer play the firewall and the other to be normal.
If thats what you want to do and your "firewall" pc has two ethernet cards then I suggest you check this out:
http://www.linux.com/howtos/Ethernet...er-HOWTO.shtml
Otherwise you need to clarify for me.