The internal web server would be as standard during your QVS installation.
You can then install just the second web server service onto another machine for external users, add this to your QMC to allow it to access your QVS.
You would then need to modify your router etc settings to allow internet users to reach the second web server, and would be a good idea to implement things like SSL.
You also have the choice of the QlikView Web server or using IIS if you prefer / depending on your skill set.
Something else to consider is how to authenticate internet users and make sure the user has the correct access to your QVW documents.
You should also review your licence agreement and the potential of using Extranet server if needed.
It's mostly a licensing flexiblity thing. With an Extranet server you loose the unlimited-number-of-documents feature but you gain flexibility in serving a large number of external users (e.g. partners, clients, etc.)
If you simply want to provide QlikView document access to employees that are mobile and already have access when connected through the internal network, I think a second QVWS (in the DMZ) is probably the better choice.