separation between the generated pulses. We also found that for our particular operation regime, the onset of the pulse splitting and the control of the pulses properties are only slightly affected by temporal variations in electron beam properties such as the current. Quadratic energy chirps do affect the spectral bandwidths and separation between the two pulses, but have negligible effect on their temporal separations. In the limit that radiation/electron beam slippage effects are small, the temporal separation between pulses can be directly retrieved from the measurement of the FEL spectrum and from the knowledge of the seed chirp properties. Such information can be extremely important when exploiting this two-pulse configuration to carry out jitter-free, pump-probe user experiments.

Acknowledgements {#acknowledgements .unnumbered}
================

We gratefully acknowledge the valuable contribution of the FERMI@Elettra commissioning and operations team. We also profited from insightful discussions with F. Bencivenga, C. Callegari, F. Capotondi and M. Labat. This work was funded by the FERMI@Elettra project of Elettra-Sincrotrone Trieste, partially supported by the Ministry of University and Research under grant numbers FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. The work of B. Mahieu, G. De Ninno and D. Gauthier has been partially supported by the CITIUS project, funded by the Program for 