Passing a variable from one program to another.

[itzpapalotl]

Noob query I'm sure, but:

I have two programs that each have a login management system. One of the programs is exclusively a login manager, the other is a scheduling program.  What I want is to log in ONCE. So.. Rather than trying to unscramble the the most heinous login scripts on either side of this equation. I figure my best bet is to modify and populate the scheduling script's login FORM page, and auto-submit it with a splash of java. All I need to do is somehow drop the user-name from the main login script some place where it can be read by the scheduling script. Here's the catch..

Each system already uses sessions, and I can not seem to drop a variable from one in to the other.  I feel I am missing something simple.

Can somebody point me the way to the idiots guide to session-handling? I tried to wade through the official PHP manual, and I feel like I am doing sessions right. I can drop a variable in to the array and read it off from another page, just not the page I need to read it off of. 

Can we start / populate / retrieve more than one session on a page? In other words can I start a third session on my homepage once logged in, which simply drops the username some place where it can be retrieved by both programs?