You can do this already. Go to your campaign that the person you want to promote is in, point to their picture, choose "Promote to GM" They will have all the rights of a GM in the VTT, and some of them in the Campaign Details page. The original GM is still the owner and there are some things only they can do, but for all practical purposes, the other person is a GM now. You can demote them from GM in the same way. If you do this while a person is logged in, they will need to refresh the page for the setting to take effect.