Major mergers between galaxy clusters can produce large turbulent and bulk flow velocities in the intra-cluster medium (ICM) and thus imprint useful diagnostic features in X-ray spectral emission lines from heavy ions. As successfully achieved by Hitomi in observations of the Perseus cluster, measurements of gas velocities in clusters from high-resolution X-ray spectra will be achievable with upcoming X-ray calorimeters like those on board XRISM, Athena, or a Lynx like mission. An interesting application to clusters involves detecting multiple velocity components or velocity gradients from diagnostic observations of specific interesting locations across the cluster. To explore this possibility in the case of a major head-on cluster merger, we perform velocity analyses of a clustercluster merger from a hydrodynamical simulation by means of X-ray synthetic spectra with spectral resolution of order of a few eV. We observed the system along two extreme line-of-sight directions: 1) perpendicular to the plane of the merger and 2) along the merger axis. In these geometrical configurations, we find that clear non-Gaussian shapes of the iron He-like K α line at 6.7 keV are expected. While the velocity dispersion predicted from the simulations can be retrieved for the brightest 100 ks pointings with XRISM Resolve, some discrepancy with respect to the expected value is noted and can be attributed to the complex non-Gaussian line shapes. Measurements in low surface brightness regions, especially when multiple velocity components are present along the line of sight, require high S/N and the larger collecting area of the Athena X-IFU calorimeter is therefore required. With the latter, we also investigate the ICM temperature and velocity gradient across the merger bow shock edge, from 20 -wide annuli extracted from a single 1 Ms X-IFU observation. For both temperature and velocity dispersion, we find best-fit values that are consistent with predictions from the simulations within 1-σ. The uncertainties on the inferred velocity dispersion are however too large to place any stringent constraints on the shallow gradient downstream of the shock. Additionally, we present simulated images of the thermal and kinetic Sunyaev-Zeldovich effects from this merging system, using the above viewing configurations, and compare the results at angular resolutions appropriate for future observatories such as CMB-S4 and the Atacama Large Aperture Submillimeter Telescope (AtLAST).