Yep, I believe it is something written in the example(s) without much thought to show how with recursive you can easily have several different WAN's connected and in "successive" failover, but for only two WAN's it is unneeded.
Ok, so, using recursive, the failover is "automatic" but in itself it only makes the Primary route (WAN1) not-active, and thus the secondary (with higher distance) becomes Active.
So you need some mechanism that checks which route is active and prevents (with this or that method), this could be a separate netwatch set of scripts.
Since you are still configuring the router, you could try changing the failover method from recursive to netwatch, so the same "trigger" will be able to prioritize the routes and disable/limit the access to the list of devices.
This is the simplest method:
Simpler Failover for two Gateways I found working - #19 by jaclaz