I have a Linksys wrt610n and this is what I had to do.
I had to go to the the cisco network magic map. Click on the router, then click set up or manage device. Put in the username and password, which both default settings are admin.
Go to application and gaming. Since you are trying to make two Xbox consoles use those ports and you are trying to connect to a server, what you need to do is to use Port Range Triggering.The ports are 53, 80, 88, and 3074 then enable. You can't use Port Range Forwarding since it will only set those ports into listening mode to the IP address you set it. So if you use Port Range Forwarding it will only be open/available to one console(the one using the IP address).
You need to use PORT RANGE TRIGGERING. Disable Port Range Forwarding (if you have enabled it for the one xbox) also disable DMZ. You need to enable UPnP as well if your Linksys router has this option (other model doesn't have this option but it is said to be enabled in default settings according to their tech support). You can find this on this router in storage and then click on media server.
To solve the lag problem set your MTU size to 1364. This settings will work even if you have one or multiple consoles running behind the router.
