I have to say I have never experienced the problem you describe. Every version of Excel I have used, including Excel 2010, allowed me to open several files and have them all in different positions on the screen. I can drag the outer edges in and out or up and down to resize them and place them side by side.

Try double clicking on the title bar of each Excel document window. This should reduce the window size to less than maximum so you can move it around.

Good luck!!!