If you enter this type of formula into these 2 cells, Excel won't be able to determine the relevant answer for either cell. It will see in A1 that the value depends upon what you entered in B1, but the value in B1 depends upon what you entered in A1. Hence, an infinite loop ("circular reference") arises.

You will need to force your users to manually enter a value in one cell only and then use a formula to populate the other cell

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Excel 2003 user (and starting to warm to Excel 2007)