NVG589 passthrough multiple static ips?

We need to have 2 routers behind the NVG589 both pulling IP's and in passthrough Have 5 useable static IP block and have passthrough setup and working fine on 1 ip, Now i need to add additional Static IP to passthrough to another router. This way we can have the same port eg. 80 on both IPs opened and working. We already went through the trouble getting it to allow another external static IP to the fixed allocation, just need to open the ports to it now.