You can not bring in a single model file and split different sections off to print with different nozzles. The dual printing process begins with the modeling process, not the slicer software. The two models need to be created such that they have a common origin and empty spaces for each other to occupy. They can not occupy the same volume. When these two models have their origins constrained using the "Dual Extrusion Merge" function, the two models are aligned together. The first model that is brought in will be assigned to the primary hotend. The second file you bring in will be default assigned to the secondary hotend.
Copy the object as desired
Scaling a dual object is also possible. If the object is especially small, it may overheat as the hotend remains confined to a given small area each layer. To give each print layer time to cool, consider printing a copy of the object that is far away from the other object. This will allow the head to come off of one object while it goes over to print the other.
