Mobile Menu Disappears When You Deselect Main Menu

I read thru a couple of threads with this subject, however the problem seems to be that whatever menu you create for the mobile menu, it will only appear if main menu is checked.

So in my case, I want a top menu on the full site, and a pulldown menu on the mobile site. In order for the mobile menu to work, I must enable the main menu, which then duplicates the top menu on the full site.

I suppose I can edit a file somewhere to pull the main menu off the full site, but I can’t figure out which file needs editing.

Thanks!