There’s no straight forward way of to use two full-featured themes for different parts of a site. There will be too many conflicting definitions in the theme files (config and layouts, assets,…)
I would go with two sites, generate them seperately to any folder you want
- public/site1/…
- public/site2/…
or
- public/main/…
- public/main/child/…
in the simplest case each of this just needs a link to the other site (pages).