I've been having an issue with forwarding the same ports to more than one device since I got a new router, which is some weird "home center" which came with the fiber connection, very strict firewall and not a lot of options to configure, although I can't replace it since both the internet and the TV comes through it.
I often host LANs with my friends where we've 4 or 5 Xbox 360s or computers connected to the internet at the same time which all need the same ports opened. Port forwarding works just fine when there's only one device connected, the NAT is open and all is well, but when a second device connects, both of them gets a Moderate connection, and then Strict as more devices connect.
I've asked my ISP for support, and they told me to use a router with UPnP enabled in addition to the "Home center", and I've got a DIR-655 connected with UPnP enabled, set to function as a switch/Access point. Yet it still doesn't work, the ports are forwarded to one xbox, which is connected through the 655 together with all the other ones.