requests from 2nd window breaks my program

I'll try to explain my problem as better as I can. Please, be "gentle" :) I developed a program for our salespeople (only them, no public) where they can order items for their customers. Order form is made of 6 pages (order info, items info, artwork, ad copies,...). Also, they use the same program to manipulate saved/submitted orders, send confirmation to customers, requests to suppliers,... Code works fine. But this morning I found a bug that if Salesperson open 2nd window in this program, since it has the same session id, it would make a mess with session. I.e., on 1st window I'm doing an order. Order ID, customer ID, transaction ID are stored in session table. In the middle of the order I open 2nd window to check a price on an other customer order. After I opened that order "new" order ID and customer ID are stored now in session table and the order (1st window) is now broken or goes to wrong place. Or, I'm sending request for approval of an order to customer. But, second before I submit the request, I open an order of other customer. Other customer ID is now stored in session table and conf. email goes to wrong place. There are few more scenarios of "bug make mess". Did anybody had a problem with "2nd window" and how is it solved? What's wrong with structure/idea I used to make a program? Any comment will be helpful to determine or give me an idea how to fix this bug. Thanks for any help. -afan