Is it possible to hide a second company from users with different roles that are not administrators?
Permissions are specific to a company, if a user is an admin they only have admin rights at that specific company.
Oh so you mean only the admin can see the other companies and not the other users?
Only the account owner can see all companies, all other users can only see the companies they’ve been added to.
You can test adding users with a single email address using email aliases, ie. firstname.lastname@example.org