We present the high-order harmonic generation spectrum from aunited two-atom system simultaneously exposed to a fundamentalTi:sapphire and its 33rd harmonic (H33) lasers by numerically solving aone-dimensional time-dependent Schrödinger equation. At a suitableinter-nuclear separation, the harmonic plateau is widened fromIp+3.2Up in the single-atom case up to Ip+5.6Up in theunited-atom case. Furthermore, the plateau is heightened in excess ofsix orders of magnitude by using the combined lasers compared with thecase of the fundamental laser alone. Such a scheme can step by stepaccomplish the following processes: through a single H33 photonresonance transition an appropriate amount of electrons are firstpopulated to an excited state, in which the electrons are easily ionizedand subsequently accelerated by the fundamental laser; then they canrecombine with the neighbouring atom, so that high-efficiency emissionsof the harmonics beyond Ip+3.2Up are successfully realized.