try a new pvc and to create a virtual wan and assign it to WAN2/ADSL (depending on what you are using). In the PVC settings you get the option to add internet connection details again.

Creating and Vlan would be a better approch though, you could add all your external IP address to the NAT pool then when the outgoing connection is made it would go from either of you assigned WAN addresses.
You can also use polices to fix what LAN address or ranges you want to go out on (Mapped IPs)

sounds feasible... will try that when next onsite. Thank you