The smoothing mechanism, that is due to the thermal conductivity in the laser plasma generated by a double pulse, is examined. Plasma preformed by a second harmonics prepulse serves as a low density gradient plasma for impacting the main pulse, the frequency of which was tripled for improving the laser-target coupling. The effect of the preformed plasma on the thermal smoothing of the heating pulse, which was split to create two foci on the target surface, was explored by varying the time delay between the prepulse and the main pulse. The smoothing effect was monitored by a pair of pinhole cameras: one viewing side-on and the other one placed at the rear side of the target. Spatially resolved X-ray emission spectra were recorded to determine the density and temperature distributions in the plasma. The maximum smoothing effect was observed for the time delay t = 0.4–0.7 ns.