Excel doesn't have these kinds of bidirectional links.
A formula in one cell in one worksheet can "pull" a value from a different
worksheet (in any workbook).
But formulas can't "push" values to other workbooks.
You can create a formula by letting excel do the work.
Open both workbooks
Select a cell on a worksheet in one workbook.
edit|copy
Select a cell on a different worksheet
edit|paste special|click Paste Link.
You'll see the formula pasted into the cell.
If you close that first workbook, you'll see how excel adjusts the formula.
