IN the early days of 29ers there were some 69 bikes with a 29" front wheel and a 26" rear wheel but reviews were mediocre so they disappeared. The 27.5" wheel size is a better compromise then the 26/29 setup.
That just applies to diameter, Asymmetric tire widths are common with a wider more aggressive tire in front for steering authority and a narrower less aggressive rear for faster rolling and better stability since the rear will lose traction before the front.