wilbur
Making a mortise that fits in the bottom of a groove is the only situation I can think of where having tools of matched widths might be of use. But even then, there are other ways around making sure that your chisel matches your plow plane blade, which have already been mentioned: making the mortise first, then the groove in two passes, or make the groove first, then use a mortise chisel that is slightly narrower than the groove, and paring the sides of the mortise to finish it off, using the sides of the groove as a guide.

It does occur to me that if you really really really want to have your tools match in width, it's probably easier to grind a plow plane blade to match the width of your mortise chisel than the other way around.