Why would you make them separate apps? Why not a single app with different sections?
Preference I guess. My thinking is that since each module may have a life of its own, it would be easier to develop and maintain the modules separately.
I am not ruling out the option of one big project. I just don't like big project. :-)
I'm pretty sure you can't share sessions between applications. I'm sure it's possible to design your app as multiple deployable applications, but my gut feel is it's more headache than it's worth.
But then again, I've only really designed one full web application. :-)
Build as seperate modules that you assemble in a single ear.