The two CAN Buses are totally separate entities, so installing the communication templates into the incorrect Bus will not work. There is enough headroom on the CAN Bus that you could have all of your devices on the same bus.
With the CAN Bus numbering, CAN Bus 1 on the M1 is an identifier for that item of hardware only, you do not need to use CAN Bus 1 on the C125 (our default configurations use CAN BUs 2) for it to work, as long as the correct CAN templates are loaded into the correct bus, then you will receive the data.