Add two themes to one site

Hello,
I am building my first wordpress site at the moment.

I am using the neptune theme as my main theme.(https://themeforest.net/item/neptune-business-theme/136243)

I have a group of seperate pages on my website that i would like to use the Retro theme ( https://themeforest.net/item/retro-portfolio-one-page-vintage-wordpress-theme/1708109?sso?WT.ac=search_item&WT.seg_1=search_item&WT.z_author=opendept)

How could i activate and toggle the two separate themes on my one site?
Is there a way i could set up the Retro theme as a template page?