Balance Two - is Passthrough possible

Is it possible to configure the Balance Two for passthrough only. I have a client that needs ISP failover between to ISPs. Can I configure straight passthrough without NATing and still have failover capabilities?
This client has two ISP’s (WAN1 and WAN2). the LAN ports 1 and 2 are connected to to two separate routers providing DHCP and Port Forwarding. At present I have the Balance Two configured to enable NAT and the correct port forwards for each secondary router( thereby creating a double NAT). I would like to know if I can get it to perform failover without the necessity of a double NAT?

Thanks in advance for your help.