I don't use multiple monitors, but it's not the multiple monitors that causes
the problem with copying and pasting formulas--it's that you have two workbooks
open in separate instances of excel.

You may want to try resizing your windows so that you have multiple workbooks in
one instance across two (or more) monitors.

Chip Pearson explains it with pictures:
http://cpearson.com/excel/MultipleMonitors.aspx

Helen wrote:

When I need to work in two separate Excel files on two separate monitors,
here's something I do:

StartRunExcel. Take that instance of Excel and drag it to one monitor
(downsize it first).
Again, click StartRunExcel; this will open a new instance of Excel.

Open your Excel files. You can now work in two separate files.

Remember: when working with dual monitors, you cannot copy/paste a formula
from one file to another.

--

Dave Peterson