Dual WAN on same ISP

[raptortek]

I currently use a City owned ISP that will allow us to have as many static IP addresses as we want.  However, they require that they are assigned via DHCP based on the MAC address.  This is the only ISP that I have ever seen do this, but their support is fantastic and I have access to the engineers in their NOC.

I have been using FortiNet for a few years with other locations. However, I wish to replace my aging Cisco ASA firewalls at our main office with a FortiGate.

For the past several years, I have had to have 2 Cisco ASA's for each static IP address for our hosted services.  We host servers that use the same ports for inbound connection requiring 2 IP addresses.  We are in the process of needing additional IP address.  My question is will the command "set allow-subnet-overlap enable" allow me to connect both WAN1, WAN2 and an additional interface configured for a third WAN into the same connection and each interface pull a different IP.  I know out of the box it will not allow it and neither does the Cisco. If I move to their MetroE circuit, the cost is about 200% the normal pricing.  Their cable is much cheaper and more reliable than Comcast or MediaCom.

My alternative is to setup a virtual PFSense firewall for the additional services and just use the FortiGate for the inhouse stuff and all the VPN tunnels and SSL VPN connectivity.  But I would perfer to do everything with the single FortiGate.

Any advise is welcome.