A theoretical analysis is presented for liquid-state T_1ρ relaxation in a coupled two-spin system I = 1/2, S=1 in the presence of two radio-frequency fields applied to each of the spins individually. It is demonstrated that the relaxation rate constant T_1ρ^-1 of the spin I due to scalar relaxation sharply increases when the two radio-frequency fields are matched according to the Hartmann-Hahn condition. Relaxation measurements on the amino-protons of 3-nitroaniline show good agreement with theory.