Is populating one cell based on its unfilled pair possible?

You would need code for that I think.

This would go in the worksheet module which you can access by right clicking the sheet tab and selecting View Code.

Private Sub Worksheet_Change(ByVal Target As Range)

    If Not Intersect(Target, Range("B2:C100")) Is Nothing Then
        Application.EnableEvents = False
        Select Case Target.Column
            Case 2
                Target.Offset(, 1).Value = 12 * Target.Value
            Case 3
                Target.Offset(, -1).Value = Target.Value / 12
        End Select
        Application.EnableEvents = True
    End If

End Sub