Connecting two FVS318v2 VPN Servers each running under a gateway routers - VPN tunnel between them

I have attempted to create site to sites like this before and the only way I have gotten it to work is to place the netgear in the dmz of the gateway router--this way everything is passed to the netgear.  Sometimes this will work and other times it will not.  

And because site to site tunnels can also fail for just small configuration errors on one or both sides, I would first take both 318s and bring them to one site and create a working tunnel between them.  You basically connect both of them to the lan the same way you have already and then each one has a 'public' ip that's on the same lan.  Then you can get a tunnel working between them (and even test throughput if you want as that's what I've used this for).  Once you have a working tunnel, you only need to move one unit and change only what you need to so it's easier to catch a mistake.

Hope this helps!