you need to have a firewall rule setup on your LAN interface to direct traffic from the computer with sabnzb through the gateway group you created that has both WANs
P.S.
you may want to bump up the number of connections to your NSP from 8, to 12.
also, 99% of binary NSP's out there dont allow more then 1 IP to be connected at any given time, so you are going to need to have 2 service providers for this scheme to work, since you have dual WAN's and each interface is assigned a unique IP