Drop-down lists - circular reference

The Data Validation List method will not update the corresponding cell value automatically. If you change the value in either cell E4 or F4 manually, the validation list dropdown in the other cell will have the single corresponding Account Code or Description and you will have to select this single value from the list to populate the cell.

If you delete either of the cell values you will see the full list in the other cell.

VBA code should update the corresponding cell value automatically.

EDIT:

Just incase I am over complicating things... is there any other way of giving the user the option to either enter in a code or select from a drop down list to give the same result?

I would probably use the Worksheet Change event approach with the following Data Validation lists:

Cell E4: Data Validation - List - Source:

=GL_Account_Code

Cell F4: Data Validation - List - Source:

=GL_Account_Description