Use of NAT with VPNs

Being a Cisc0 ASA man for years, I am new to Fortigate,I have just configured my first Site-to-Site VPN.

I had to use NAT to hide the real remote site IP from Local. Traffic can be initiated from either end.

this is the requirements

local 10.50.0.0/24(local) to 10.150.34.0/24(remote-NAT) NATed to172.18.36.0/24(Remote) on Fortigate

so tunnel comes up ok using Local to Remote.

Firewall rules

outbound  Local to Remote-NAT( using virtual IP to translate to Remote)

inbound Remote to Local ( using IP pools to translate to Remote-NAT)

This all works, but my question is

On Cisco you only require one NAT rule which will cover both inbound and outbound.

but it looks like on Fortigate I need two one defined in virtual IP and the other in IP pools.

AM I doing something wrong?