Heating of the ion component of a radially non-uniform plasma cylinder under the condition of double ion gyroresonance is studied in the quasi-linear approximation. An equation is derived for the diffusion of ions in velocity space; the radial distribution of the absorbed power is also obtained. The thermal energy of the ions is shown to increase even in plasma regions where the work of the electromagnetic field done on ion electric current is negative. The efficiency of the method of ion heating under the condition of double ion gyroresonance is compared with that of the method of "magnetic pumping".