I have set up a pair of gateways for a similar scenario where the provider gave me /30 and an ethernet jack instead of providing a router on-premises. This is what I did:
-Configured an interface on each machine to come up with no IP. -Configured a carpdev to use the no IP interface on each machine. -Configured my ip from the /30 on the carpdev on each machine. Other things included CARP on other interfaces like LAN and DMZ. In my case those IP networks were large enough to allow me 1 CARP IP and an IP for each gateway. Not sure if that helps, but the best general advice is to draw a picture of what you want. Read the FAQ/manpages to draft a config. Test all that, and if you are like me, realize you didn't really want bridge at the one place in the drawing and revise--repeat. Good luck!
