Moving Apps from laptop to desktop

Hey,

I have both a laptop and a desktop, and I've downloaded apps using my iPhone, my laptop and my desktop. When I try to update one set of apps it deletes the other set. How do I get iTunes to automatically save apps onto either my laptop or desktop from my iPhone regardless of how I made the initial purchase? I tried just copying the files from my laptop to desktop via firewire but it didn't seem to work. Please help! I've spent a very frustrating morning going back and forth between the two.