There is a way but it needs to be done in the command line, not the GUI (and apparently not on the xxD versions of the FortiGates (having that problem now).
What you need to do is:
config system interface
edit interface name
set ip ip address/subnet (use a /32)
set remote-ip ip address
end
Do the same thing on both sides of your VPN. Those addresses are your neighbors.
for example:
Side A:
config system interface
edit side_a
set ip 10.40.10.1/32
set remote-ip 10.40.10.2
end
Side B:
config system interface
edit side_b
set ip 10.40.10.2/32
set remote-ip 10.40.10.1
end
Now you can go into your BGP setup and make your neighbors the other side' s IP address.
It appears that this solution can not be done on, at least, the 60D FortiGates because as soon as I hit end on the 60D config, the system goes into a continuous process of rebooting, until I unplug the internet port.
If anyone has come across a solution to this, please let me know. I need to roll out some equipment and would rather have seven ports than five.