I honestly hate to give bad news but this is really related to end-user customization which isn’t really part of support. I’m also not familiar with that multiple themes plugin, so how this thing works, I do not know (interesting though). I’m trying to figure out what kind of a setup you are wanting to do, but it sounds extremely complicated and never heard of someone trying to do what you are attempting. Have you done this before with other themes?
Please note that each theme has different layouts and structures, as well as css and functionality from each other, so I think you will find this will be hard to achieve. I would believe a multi-site option is the better solution, especially when there are theme updates, WordPress updates, and plugin updates that could cause more headaches down the road.