From what i understand, you want to connect FGT (2 ports) - 1 port connecting to FSW1 and 1 port connecting to FSW2. Both switches managed by FGT.
FSW1 connected to ISP1 and FSW-ISP2.
This can be achieved. You can use interface type as "hardware switch" on FGT https://docs.fortinet.com/document/fortiswitch/7.0.4/devices-managed-by-fortios/801202/single-fortig...
Could you please send the documentation link where it says to connect ISL.
Wifi-Switching – International Support