So N-1 is pressed on. When N-2 is pressed N-1 turns off. When N-2 is on and N-1 is pressed on, N-2 turns off.
Manual says "Both IF's are cascaded such that the resulting receiver bandwidth
is a combination of both filters. See page vii for a chart of typical
bandwidths an shape factors for various combinations of filters."
Keep The Faith, Jim K9TF/WA9YSD