http://answers.ea.com/t5/PvZ-Garden-Warfare-PC/2Pcs-2origins-2PvzGW-cant-play-together-y/m-p/3331949#U3331949
Maybe just try disabling uPnP and power cycling the router for a few minutes.
Check router for a firmware upgrade. If the router supports Custom Firmware like DDWRT you might consider giving it a try. Some of my friends have multiple Battlefields going in the same house and they use DDWRT.
We use a FreeBSD firewall as a router and it lets us play all our games together. Some extra settings needed flipped around that most other routers don't have I doubt (like static ports).