Sorry, your post slipped by...
Why dial-up? Just let the branch office FGT initiate the tunnel. One side is static IP, one side is dynamic IP/DDNS. These days (i.e. in FOS 5.2+) Fortinet offers a free DDNS account for FGT owners.
Using a static IP and a dynamic name, you can even use Main mode.
BTW, even with your setup you should be able to send traffic in both directions. Probably one of the 2 routes necessary is missing, or one of the policies. Check the routing table on both FGTs (Routing > Monitor) if there's a proper route for each private network.
IIRC, the FGT will create a host route for each dial-up client (a /32) dynamically on connect. This would then not work for 2 subnets. You'll see that in the Routing Monitor.
Again, drop the dial-up idea altogether.
post edited by ede_pfau - 2017/03/08 07:42:37