I believe the issue is that you still can only select 1 alternative form, so whichever is chosen will show. The visibility can help toggle what fields show from that one form you selected.
What you want to do is put 2 forms of the page (in the page template using online for web part), set one form to the first, and the second form to the alternative, and add a macro to the visibility field of the web part so one shows for one device profile and the other shows for the rest.
Or just make 1 form and use bootstrap or similar to morph how the form looks on different devices that way, easier.