wordpress under wordpress

hello

i want to make wordpress page but got to problem about two themes.

i will use as main theme – hueman but
secondary https://www.wpsolver.com/ideas-customer-feedback-theme-for-wordpress/

problem is in that the second theme those “ideas” create as posts but if i will post – they will appear in main where i don’t want to because there will be a different posts.

got idea about make two wordpress on server but again:
i need one register and login system on both wordpress not register each wordpress.
and second – it will spend space double.

i like in second theme the front end page – button to submit + how they appear + widgets for easy search but this doesn’t fits in main wordpress.

as idea i take multiple theme but dont works as i said:
main site – diferent posts
secondary – diferent posts
both in common – login and register system that no need to register in each wordpress