When you connect the main display to that video card, then the Mac OS will make that your default card. I don't think there's a way to connect all your displays to the 580X and then assign the other GPU as the "main" processing card outside of the actual application that can make a selection directly. I could be wrong.
What I do is connect my multiple displays to the two video cards. That means you will have to invest in a USB-C to DisplayPort adapter, which I've done. Once you've done that, to switch to the "main" video card, you simply re-arrange the menu bar under the Arrange Displays setting. You can verify which "main" video card is being used when you click on "About This Mac."
I'm at the office right now and using my laptop, so the screenshot I attached will be slightly different but the same idea.
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