Timing synchronization between two independent Ti:sapphire mode-locked lasers has been developed for Laser-Compton X-ray generation. The lasers operated at different repletion frequencies of 119 MHz and 2856 MHz. The two lasers were actively synchronized with a phase-locked loop at a frequency of 2856 MHz. Fluctuation of the optical sum-frequency intensity was measured, and timing jitter was obtained. The rms timing jitter between the lasers was below 5 fs for several tens of seconds. The fluctuation was measured with a maximum observation bandwidth of 300 kHz. To improve long-time stability, a sum-frequency signal was fed back with a slow loop bandwidth, achieving long time operation for 1 hour with 5 fs synchronization.