Excel 2003: Side by side windows. An excellent new feature, very good.

But if you resize one of the workbooks, the other should resize
automatically. One gets bigger the other smaller.

What if more than 2 workbooks open, choose 2 (or 3 or 4?) which resize
automatically and the remaining workbooks do not change size. I often have 2
open on the screen, so they should resize automatically.