The current study investigates the phase evolution during multi-material printing of a CuCrZr alloy and an Inconel 625 nickel-based superalloy via operando X-ray diffraction. Initially, two samples were printed using Inconel 625 and then a CuCrZr alloy was printed on top using two different process parameters. To follow the phase evolution during LPBF, operando synchrotron X-ray diffraction measurements were conducted in transmission mode. The patterns were collected over successive layers around the interface during both melting and solidification. It was shown that two separate FCC phases formed upon melting and solidification despite the expected complete solid solubility between copper and nickel. Further investigation found that this behavior originates from the thermodynamic behavior of minor alloying elements within the two alloys. These results provide a deeper understanding regarding the solidification behavior of multi-materials during LPBF, and hold promise for further optimization of multi-material components produced by AM processes.