I will setup wordpress multisite.
Site-1: www.abc.com (is the main site).
Site-2: www.abc.com/site2 (is a subsite).
I would not set the other site in the sub directory as a subdomain.
Right now if a user registers and login to site-2 then he is registering on www.abc.com and can access both the sites.
I want to restrict users on site-2, so if they visit site-1 they will not be logged in for that site.
Site-1 also has some user roles. Administrator, Business owner, Installer employee etc. Business owner should have permission to log in to site-2 and only he can allow his team (installer staff) to log in to site-2.