I have 2 pptp internet connections (aplus1, aplus2) and 2 internal networks (192.168.10.x, 192.168.11.x).
Each pair separate from another in firewall (drop forward) and forwaded using mange connections and routes.

BUT i need correct working UPNP, i mean that UPNP have to use exact external interface for exact internal network. aplus1 -> 192.168.10.x, aplus2 -> 192.168.11.x.
Now it does as it wants - when i run torrent, it may open ports from aplus1 to 192.168.11.x and vice versa.

ROS doesn't have that feature, but depending how you are using your WAN connections, you may be able to use meta router as a work around.