To do as you wish will require a VBA solution. This example uses the cell references in your example comment.
Copy this event handler to the clipboard (highlight the entire event handler, right click inside the highlighted area, and 'Copy')
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address(0, 0) = "B5" Then
Range("B6").Value = Round(Application. Convert(Range("B5"), "m", "ft"), 2)
ElseIf Target.Address(0, 0) = "B6" Then
Range("B5").Value = Round(Application. Convert(Range("B6"), "ft", "m"), 2)
Select the worksheet containing the data you wish to convert and right click the sheet tab at the bottom.
Select 'View Code'.
Paste the event handler into the white editing area to the right (right click inside the area and 'Paste').
Close the VBE (red button - top right).
Save the workbook as an Excel Macro-Enabled Workbook to retain this functionality for future use when the workbook is opened again. Archive, or delete, the original version (type .xlsx) in order to prevent confusion as to which to use.
Enter the value 100 in cell B5 and cell B6 will display 328.08 (rounded to 2 places). Enter 200 in cell B6 and cell B5 will display 60.96.
This solution is guaranteed to function exactly as you specify in your question.