The stability of the SwissFEL photoinjector laser is crucial for stable FEL operation in the hard X-ray regime. In the 10 pC operation mode, in which sub-10 fs photon pulses will be generated for the users, the arrival time jitter of the laser pulse at the photocathode should not exceed 40 fs (rms). Therefore, it is foreseen that the titanium-sapphire gun laser oscillator (Ti: Sa) is stabilized by optical cross-correlation with the pulses from the laser master oscillator, which is an erbium-doped fiber laser (Er:FL). In this paper, we demonstrate a stable optical lock for 60 min. between two such mode-locked oscillators, which were placed on different optical tables and connected optically but not mechanically via a 5 m long free-space transfer line. Furthermore, we make a comparison between the RF- and the optical lock stability of the slave laser in the frequency range from 10 Hz to 10 MHz.