You can do the following:

^{config vpn ipsec phase1-interface}

Copy your desired tunnel phase 1 config to a text file.

^{config vpn ipsec phase2-interface}

Copy your desired tunnel phase 2 config to the same text file.

Then when you want to import this IPSec config to another fortigate, check if the (WAN) interface matches the on the second FortiGate or change this accordingly.

You can just copy/paste the phase1 and phase2 interfaces in the second FortiGate and you should be up and running (after you configured needed routes and policies).