dontknowme on "Multiple Themes for Multiple Functions."

Hi, I was wondering what exactly this plug in does. I understand this theme allows to use different themes for different pages etc but does it allow you to use the different theme septic functions also? As in, if I had one theme which was a forum theme and another theme that was a classified ads theme. And if a user registers on my site on the forum, can they use the same account to sign in on the classified theme to post an ad? Do the users works between the theme and work well?

Is it likely if I use multiple themes for several functions, the users can support the different features. I want the users to only register once, but be able to post in forums, post classified ads (another theme), buy items from shop (another theme)? Sorry if this is not very clear.

Thanks

https://wordpress.org/plugins/jonradio-multiple-themes/