  We theoretically study the scaling with the driving wavelength of the
high-order harmonic generation (HHG) under the simultaneous irradiation of an
extreme ultraviolet (XUV) pulse. Surprisingly, when the cutoff energy and
ionization yield are fixed, the harmonic yield is nearly independent of the
driving wavelength. We identify its origin as the combination of the initial
spatial width of the states excited by the XUV pulse, making the wavepacket
spreading less prominent, and the shallowing of the ionization potential, which
suggests complex nature of the wavelength dependence of HHG.
