2 sites on the same path

I have this situation where in need to provide external access to some users for a Sharepoint 2010 site. The external authentication will require certificates and the connection will be HTTPS, while the users from inside the company will authenticate only with user/pass and the connection will be HTTP. So i need two different ways to authenticate them into the same Sharepoint site.

I was thinking to create 2 sites in IIS, one on port 80 and one on 443, each with a different authentication method. The problem is i can't figure out how to do this. Please help.