When setting up a pair of Watchguard boxes to do a site-to-site VPN, there is no "which dials which". Each end is set to search out the other at a specific IP (or FQDN if you have dymanic IP's). They find each other and connect just a few seconds after booting up.
Start by reading this: FVS124G Reference Manual. I just did, and found out that the router has a fairly-straightforward wizard for setting up gateway-to-gateway VPN tunnels. You just have to enter the remote WAN and LAN addresses, and the PSK.