Cool.

2 methods (I prefer the second)

1   Open 2 instances of Excel, each maximised in a different screen.  This
is less flexible as these instances will not "see" each other.
2   Open only one instance and span it across both screens (likely to need
to "restore" rather than "maximise" and then manually resize (strech) it to
size).  Then, open a couple of workbooks, click the "Window" menu and tile
them.  You can then resize and move the different windows around.

I've just noticed something.

You actually can have different windows of one workbook.  (I.e. you can do
exactly what you want to do).  To do this:
Open your workbook.
Span excel across both screens
Click Window, New Window
Click Window, Arrange, Tile
You now have two windows of the same sheet which you can browse and resize
as much as you want.

Hopefully this'll do the job.  Reply if any difficulties.