I finally got this sorted out. Here's how I have done, in case anybody in that situation happens to find this thread. This method does not require creating NAT outbound rules.
Assign the WAN2 interface with DHCP or static. This is the WAN of my additionnal public IP
Create a VLAN and assign it (I'll call it LAN2 for clarity). VMs using the additionnal IP will be connected to this VLAN.
Go to LAN firewall rules, edit the default IPv4 allow rule, and set the gateway to your WAN gateway.
Go to LAN2 firewall rules, edit the default IPv4 allow rule, and set the gateway to your WAN2 gateway.