Mouse going oveer dge of Xinerama screen 1
 

Hello everyone,
I have set up a dual-monitor system which has one 1280x1024 screen (Xinerama screen 1) to the left of a screen measuring 800x600 (Xinerama screen 2). The bottom edges are aligned, effectively forming something like an L-shaped screen. This setup works as it should, except that should I want to reach the 'X' button in the top left of screen 1, I need to take extreme care not to let my mouse slip over the edge of screen 1, because if it should, then it appears at the top-left of screen 2, which is 480 pixels further down.
It's not a lot of work to move the mouse cursor back up, but it's very annoying. Should I move the cursor off the top edge of screen 2, then it does not move onto screen 1, and I would like the top-part of the rightmost edge of the screen on the left to behave the same way. Does anyone know how to fix this?

Apologies if I haven't explained the situation very well.