Dear All,
Saw closed question in a thread and decided to post.

To have a fail over with priority interface only 1 UT APP is needed: Failover

In previous question, there was decision to have Wan Balance with 100% to one interface and 0 to another.
This is not needed

As by 2014 post from Dmorris:

The lower the ID interface (in config > network > interfaces) the higher priority.
The lowest ID live WAN is the active WAN.

So you just need 1 APP fail over, you should configure interface With ID 1 (Or by you setup, WAN with lover ID is in priority ) with your preferred ISP

Tested on Version 14.0