RRAS with 2 incoming NIC

I have a server which host exchange server 2003 with 3 NIC, 1 for internal LAN and the other 2 for incoming connection like VPN and remote access.

Both external NIC are using fix IP (NIC A and B). How am i going to let all incoming connection uses the NIC A instead of B now. I want NIC B for backup as it is a ADSL.

Should i change the default routing to point to the NIC A? Or is there any other option.