Debian – How to setup a backup internet connection from a second ISP

Oh dear... I think you're trying to use the Xen bridging stuff, which is just dire. Set network-script network-dummy in xend-config.sxp, then go with this config:

iface lo
iface lo inet loopback

# Local network, cable labeled M3
auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.184
    netmask 255.255.255.0
    gateway 192.168.1.1

# cable labeled M1
iface eth1 inet manual
    hwaddress ether 00:19:5B:33:86:D5

# cable labeled M2
iface eth2 inet manual
    hwaddress ether 00:19:5B:33:86:D3

auto br-eth1
iface br-eth1 inet manual
  bridge_ports eth1

auto br-eth2
iface br-eth2 inet manual
  bridge_ports eth2

What you then do is tell the domUs to use either the br-eth1 or br-eth2 bridge (as appropriate). Given that you've got cable labelling happening, I'd change the bridge names to something more useful, like perhaps m1 and m2.

It sounds like you're looking to use one of your static IPs as the main public IP that outbound Internet traffic for your office uses, and then map a separate one of your public static IPs for use by the server? If that is the case, you'll just want to create a "one-to-one" NAT entry in the firewall. Using the emulator link you provided, it would be under the Setup -> One-to-One NAT. This basically associates one of the external IPs to an internal IP address.

Assuming your ISP is providing you a "bridged" connection (so that you can assign the public IP info directly to your firewall), you'll basically plug in the IP info into your firewall. Assuming you can access the Internet, the router will typically route all traffic outbound via the IP address you assign to it. If you have one-to-one NAT enabled, those hosts should communicate via a secondary IP.

Does that answer your question?