This illustration shows how wavelength y of twice the length of wavelength x will have the same angles of order as wavelength x . (That's the 1st, 2nd, 3rd, etc diffraction orders)
If the longer wavelength y is at twice the speed, then the incident frequency at any incident plane parallel to the diffraction grating will also be the same. Effectively making the two separate wavelengths appear to be the same wavelength at the diffraction grating.
