Modern engineering alloys contain multiple alloying elements, but their direct observation when segregated at the atomic scale is challenging because segregation is susceptible to electron beam damage. This is very severe for magnesium alloys, especially when solute atoms segregate to form single atomic columns. Here we show that we can image segregation in magnesium alloys with atomic-resolution X-ray dispersive spectroscopy at a much lower electron voltage. We report a co-segregation pattern at twin boundaries in a magnesium alloy with both larger and smaller solutes forming alternating columns that fully occupy the twin boundary, in contrast to previous observations of half occupancy where mixed-solute columns alternate with magnesium. We further show that the solute co-segregation affects the twin migration mechanism and increases the twin boundary pinning. Our work demonstrates that the atomic-scale analysis of the structure and chemistry of solute segregation in metallic alloys with complex compositions is now possible.