He already did. Megapixels is one form of measurement that encapsulates both dimensions into a single number. Technically for a sensor this would be a measure of sampling rate which influences, but is not necessarily, final resolution. But you can describe captured images in terms of MP as well. And while I abhor the way DxO abuses the term, you could describe images with the same storage dimensions in terms of "equivalent megapixels" or "resolved megapixels" as compared to a reference, or hypothetically ideal, target.
As for printers, it can be important to know if a printer has discrete resolutions in the horizontal and vertical dimensions, and manufacturers state this if it's the case. Which again betrays the fact that we typically throw around a number for a single dimension out of convenience, but we are dealing with a 2D medium. And that 1D value must be applied to both dimensions to understand how much data is captured.
Another example I can think of where one dimensional measurements can be misleading: lp/mm tests can overstate system resolution because a line, being amplified along one direction, is easier to detect than a detail closer in shape to a dot. That's less familiar to photographers than the fact that resolution can vary with line pair orientation to the system which is why lens tests include the diagonal. All of this betrays the fact that yes, we measure along one dimension for convenience, and do so repeatedly at different orientations for more accurate modeling. But at the end of the day the medium is 2D. A 24mp array can provide 20% more data points than a 20mp array, assuming no other limitations on the system.