Hi RigoV,
Welcome to the Duo Community.
I have no elegant solution for this. Much like the way you can define what application is used to open a .pdf file in Windows. It depends on iOS or the Android OS adding functionality for you to be able to define a default authenticator application manually. This would be the same way you define a default browser on your phone to use Chrome rather than Safari for example.
From my research such functionality both in iOS and Android does not yet exist at this time, and you will either need to remove the application you do not use, or live with the extra steps.
From your test results so far, I suspect iOS is simply using the last authenticator application installed as the default, but i have not tested this my self to confirm.