QIDI Tech dual extruders

Hi. I use Fusion 360 to prepare models for my QIDI Tech 1. I am trying to make models that can be printed in two colours using both extruders. I have followed tutorials in which separate STL files are created for the separate extruders, but none of the methods works. The STL files load separate models onto the QIDI plate. As soon as I use Merge or Group, the separate models combine into a single model. I cannot then access the parts in order to assign them to a specific extruder. Does anyone know how to do this?
Thanks, Joe