How to create 1 Wan with 2 Lan Trunk

unfortunately your description is hard to follow. Can you add some diagram? 

I have FortiGates here that have three WAN with two different providers and either NAT behind the WAN (where is the Router that has the ADSL Modem) or WAN IP directly on the WAN of the FGT.

I handle them all via sdwan. So they can be in an sdwan zone alltogether.Can have some health checks and the rest can be done either with policies or/and sdwan rules.

I took the time to draw it and I'll share it, thank you in the meantime

How it is possible to use 2 different IP/DG on WAN2 interface?
outside there is another Firewall that have the 2 vlan 

ok one port can not be a trunk on a fortigate. You would have to create two vlan interfaces that are tied to WAN2 on your FGT. Then you have to have a default route and static routes for what you need.

However you can only have more than one default route for redundancy (and that only as long as you don't use sdwan). The metric (prio/distace) will set which default gw will be used. In case you use sdwan you just need a default route via sdwan and sdwan does the rest for you.

So what do you want to route to where?

WAN1 natted to internet only? WAN2 used to access the two vlans behind the other firewall?

Wan 1 is natted and used in a dedicated lan of customer, wan 2 only to access to the two vlans. 

no redundancy or communication between networks is needed, they are two distinct networks, one serves wan1 while the other relies on two vlans

BTW: if you create the two vlan interfaces on WAN2 with correct vid and each has an ip out of the corresponding vlan you don't need static routes because in this case there is connected routes coming with those interfaces. In that case only the other side would need to have a route back to you to be able to answer your requests.

ok thanks, I'll try it as soon as possible, for now I've configured them like this

yes that's what I said. Two vlan interfaces tied to WAN2 and both have an IP out of the corresponding VLAN.