two theme on one WP

Currently i have one wp (Not multisite) with one theme (Theme A) and i want to add another theme to that same website which we call (Theme B) so how can two theme in one single WP?

The only workaround i can think of is import the WP into my Multisite. Therefore i have 2 Subsite with two different theme. But mapping will be issue. How do i map: - Theme A - Theme B (i would prefer but i dont think it is possible)

The purpose for this is theme A is just for simple blog and theme B is used to host tons of vimeo video. and i cant find suitable theme that cater for both usage.