No, you are not required to set them to the same letters. This allows for 16 different combinations. However, for practical reasons, setting them both to the same tends to make the most sense for most of us. Plus, the spreadsheet assumes the two will match since that is the way it is setup.
That said, the spreadsheet is just a set of guidelines or a good starting point. Experiment with the settings and see what needs to be tweaked to match your shooting style.
---------+---------+---------+ Joseph K Seattle, WA, USA