How to edit alternate inactive themes in “Appearance -> Customise” screen?

While it is possible to have two themes being “active” together, it is not a setup anyone is going to support, and you are going to be “step into walls” time after time.

As an easy example, you will have harder time configuring caching, if it will be possible at all.

Do the right thing and make your current theme responsive, or switch to a better one.