My standard advice is to create the MU and place the engines say an engine length apart. Start running and observe how they behave. The distance should not vary by much. You may have to change not just the top speed but the starting speed and every step in between as well.

This very good advice. If the two locos are identical they they should probably have the same performance parameters but if they are not you will need to customize their whole performance envelope for them to run together well. That may not be the easiest thing to accomplish and it will take careful attention to detail to make that work correctly. I normally only make an MUC out of models that are mechanically identical and even then they do not always pull together harmoniously, especially when taking into consideration the starting delay caused in newer locos when the operating sounds are activated. My croc double set runs perfectly together without operating sounds but occasionally one loco will start a bit before the other when the sound is turned on.

Good luck and keep us informed on how this goes since it could be instructive for all of us.