Running Multiple Applications with two CodeIgniter Installations

[eluser]gojo[/eluser]
I have been thinking about this all day I would love any coherent advice on the subject.

I have two facebook apps one for writers and one for readers in test mode at the moment and for simplicity they have two different localhost addresses
https://localhost/readers
https://localhost/writers
but I have decided it would be better to have one website when it goes live say
https://www.books.com and have the two facebook apps in seperate codeigniter installations so one in
https://www.books.com/readers and the other in https://www.books.com/writers that way they can share the image directory etc but have seperate sessions.
My brain aches from trying to work out how to do this can anyone help explain what I need to do and will they be able to share the images and have seperate session controls?
Thanks in advance if anyone can help gary