Yes, that's exactly what he asked.
When you create Internet Flow Preferences you can specify a prefered WAN interface. However, if that interface fails then the rule will fail over to the other WAN interface, if it's available.
Your logic is completely wrong. Your rule has a destination IP of the WAN interface. Client traffic will never have a destination of the WAN IP. If I'm trying to reach Google, my packets don't have a destination of my WAN interface, they have a destination of Google's IP. This means that if you specify the WAN IP as the destination in an ACL it will never match anything.