Multi Pilot is a property of an Aircraft Type.
When you create or modify an aircraft type, use the toggle for "Multi Pilot" to enable or disable it. If the specification is not shown, tap the toggle icon in the top right hand corner of the Aircraft Type list screen to switch it on.
Whenever time is logged and Multi Pilot spec has been switched on, it will be attributed to the correct column in the reports.
Please note that RB Logbook does not have a Multi-Pilot field to log times in a flight, like many other logbooks. The way we have set this up with the aircraft type is more convenient because it makes logging it separately unnecessary.