#Fortigate Explicit Proxy Routing

Hi I configured an Explicit Proxy on a Fortigate appliance. Fortigate use FSSO for remote users authentication and it has tree main interfaces: 1 LAN interface on which explicit proxy is configured and 2 WAN interface. My goal is that User A proxed traffic flow through WAN1 interface and User B proxed traffic flow through WAN2. I know that it's not possible use PBR and my customer doesn't want use an additional vdom to re-route traffic generated by Proxy. Is there another solution to achieve it?