yes do not use overlapping subnets. That causes too much trouble and obfuscating.
Build the Site2Site as said. Create the policies on both sides to allow traffic to flow (ipsec will not come up without policy anyhow) and also make sure that both side have route to the opposite subnet that should be reachable.
you could always run a flow trace on cli on either FGT to see what happens to your traffic:
diag debug enable
diag debug flow filter clear
diag debug flow filter <filter> (without paramters it will show all current filters or "?" to show parameters)
probably set a source and/or destination address to filter (because without you will get tons of traced traffic)
diag debug flow trace start <numberofpacketstotrace>
then to some ping from some client to some device that uses the vpn and see what the trace says
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