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Split-pulse x-ray photon correlation spectroscopy has been proposed as one ofthe unique capabilities made possible with the x-ray free electron lasers. Itenables characterization of atomic scale structural dynamics that dictates themacroscopic properties of various disordered material systems. Central to theexperimental concept are x-ray optics that are capable of splitting individualcoherent femtosecond x-ray pulse into two distinct pulses, introduce anadjustable time delay between them, and then recombine the two pulses at thesample position such that they generate two coherent scattering patterns inrapid succession. Recent developments in such optics showed that, while true'amplitude splitting' optics at hard x-ray wavelengths remains a technicalchallenge, wavefront and wavelength splitting are both feasible, able todeliver two micron sized focused beams to the sample with sufficient relativestability. Here, we however show that the conventional approach to specklevisibility spectroscopy using these beam splitting techniques can beproblematic, even leading to a decoupling of speckle visibility and materialdynamics. In response, we discuss the details of the experimental approachesand data analysis protocols for addressing issues caused by subtle beamdissimilarities for both wavefront and wavelength splitting setups. We alsoshow that in some scattering geometries, the Q-space mismatch can be resolvedby using two beams of slightly different incidence angle and slightly differentwavelengths at the same time. Instead of measuring the visibility of weakspeckle patterns, the time correlation in sample structure is encoded in the'side band' of the spatial autocorrelation of the summed speckle patterns, andcan be retrieved straightforwardly from the experimental data. We demonstratethis with a numerical simulation.

Realizing split-pulse x-ray photon correlation spectroscopy to measure ultrafast dynamics in complex matter