Running 2 sites within one

Hi,

a client has requested a site which is effectively 2 sites within 1 (though both would share some content) with a portal page as the home page allowing you access to either site. I’m wondering how best to achieve this in wordpress? Whether i can have all the content in the one site, with each side’s content hidden from the other (which would require 2 menus, each one belonging to it’s respective content) or whether i would have to have two wordpress installs, 1 per site, and then replicate the content that is needed on both sites?

I hope this is clear and hope someone may be able to advise

Thanks