You need to identify each pair of hot wires with there corisponding neutral first try starting at the romex sheath in the panel with the power to the panel OFF and follow each to there termination and label them. And relocate breakers as necessary. If this is not possible it is much more difficult .If you can find the first box that each MWBS is run to you can use a breaker finder tracer and a continuity tester to locate each wire at the panel. Tracer for the hots and continuity for the neutral with the wire taken loose at both ends. Continuity test needs to be done with all power off. Continuity can be used for all wires and is easiest with 2 people and a 2way radio or cell phone. If you aren't sure of how to test continuity tell me and I'll explain.