## 3 Results and Discussion  
To understand the benefits of acquiring two spectra in-parallel, we consider the measured spectral intensity on the detector \(S(\omega)\), which depends on the photon frequency \(\omega\) for each beam, to be proportional to the spectral reflectivity \(R(\omega)\) of the sample, the diffraction efficiency \(O\) of the beam-splitting grating in the zeroth or first order and the spectral intensity \(I(\omega)\) of the FEL:  
\[S_{1}(\omega)\propto R_{1}(\omega)O_{1}I(\omega)S_{2}(\omega)\propto R_{2}( \omega)O_{0}I(\omega) \tag{1}\]  
It is apparent that the measured spectrum \(S(\omega)\) of a single beam can be used to measure the reflected spectrum \(R(\omega)\), if the FEL intensity \(I(\omega)\) is either known or constant. If many pulses are averaged, SASE fluctuations average out, but potential slow drifts of the average pulse energy remain. Since the third harmonic radiation intensity scales roughly with the square of the fundamental [17], slow fluctuations are mitigated by normalizing each spectrum with the square intensity measured by the XGMD which monitors the fundamental radiation of the FEL further upstream. This leads to the FEL spectra shown in Figure 2a,b, each pair was measured in-parallel using \(10^{5}\) SASE pulses within four minutes. Synchrotron spectra of reflectivity and drain current recorded on the same sample with a similar angle of incidence (\(\theta=11^{\circ}\) for the O \(K\)-edge, \(\theta=12^{\circ}\) for the Ni \(L_{2,3}\)-edges) are shown for comparison and demonstrate a good agreement to the FEL spectra. The FEL-reflectivity spectra recorded at these angles reproduce the relevant structures of the drain current absorption spectra. We ascribe the minor differences to sample inhomogeneity and the difference of the incident angle. Especially the O \(K\)-edge spectra also show signs of surface contamination with water (especially the structure above 545 eV), as the sample was exposed to air both before and between FEL and synchrotron measurements. In the following, we analyze how accurate changes in one spectrum due to FEL fluctuations can be used to measure the changes in the other spectrum. Our main motivation for recording two spectra in-parallel is to exploit one of them as a spectrally resolved intensity reference in a pump-probe scenario. In this case, a crucial parameter is the precision with which (pump-induced) changes in one spectrum may be determined from the other. The fraction of fluctuation that cannot be explained from the variations in the reference spectrum is quantified by the so-called fraction of variance unexplained (FVU). The FVU for a set of \(N\) pairs of spectra equal unity minus the coefficient of determination \(r^{2}\) between both spectra, which is in this case, the square of the Pearson correlation coefficient \(r\):  
\[r(\omega)=\frac{\sum_{i=1}^{N}\left(S_{1}^{i}(\omega)-\overline{S}_{1}(\omega) \right)\left(S_{2}^{i}(\omega)-\overline{S}_{2}(\omega)\right)}{\sqrt{\sum_{i =1}^{N}\left(S_{1}^{i}(\omega)-\overline{S}_{1}(\omega)\right)^{2}}\sqrt{\sum_ {i=1}^{N}\left(S_{2}^{i}(\omega)-\overline{S}_{2}(\omega)\right)^{2}}} \tag{2}\]  
\[FVU=1-r^{2} \tag{3}\]  
Here, \(\overline{S}\) denotes the average spectrum over the set. We find that minor alignment imperfections and pointing drifts can lead to a small offset along the dispersive direction of the detector between two simultaneously recorded spectra. Similar to previous work [15], this offset is determined by shifting the reference spectrum along the dispersive axis (see Appendix A for details) such that the logarithmic FVU, integrated over the entire spectrum, is minimized. The offset is determined in this way for every measurement (each in the order of several minutes) separately, yielding a shift between one and three pixels, i.e., some tens of \(\upmu\)m on the CCD detector.