The ability to assign objects to processes is very powerful, but it's too hidden in the GUI. This makes it hard to know that you've got objects associated to processes properly, because you have to drill down into each process' configuration to check them.
I'd like to suggest that processes be indicated by colors in the preview. So, for example, doing dualstrusion, you could see by it being red (for example) that one STL is associated the left extruder process, and the other STL by being blue is associated with the right extruder process. This should also make it obvious if an STL was not associated with any process, or multiple processes.
I would also suggest that the command to associate an STL to a process be added to the objects properties dialog, and perhaps a right-click menu on the object, to allow for direct manipulation.
Similarly, in the G code preview it would be very helpful to differentiate between left and right extruders by color. Currently color is used to indicate temperature, but perhaps one color axis could be used to indicate extruder (red/blue) and another axis to indicate speed (light/dark)?