Multiple sites with one instance and a subdirectory twist

Scenario:

I have a Contao instance intalled in domain.com/dir
I have another site I would like to reside in domain.com/dirfoo and I would like to be able to administrate it with the previously mentioned Contao installation.

Question is, how? Any input would be highly appreciated.

You can install Contao in the root folder and then create different root page for each site

eg.
in domain.com/dir the actual installation

domain.com/dir/site1 the alias of the root page of site 1
domain.com/dir/site2 the alias of the root page of site 2