Flashforge Creator Quick Start Manual page 18

You'll see three fields: one for each extruder and one to name the merged file. The
fields for the two extruders will accept either .stl files or Gcode you've already
generated. The merged file will be Gcode, so give it a .gcode file extension.
When you click "Merge" two windows will pop up -- one to generate the Gcode for
each of your two component parts. These will look a lot like the Generate Gcode
window you've used for single extrusion. Make sure to check "Use default start/end
Gcode" and "Use Print-O-Matic". You'll also want to make sure that "Use Raft" is
unchecked for both items -- rafts currently don't work well with DualStrusion prints.
After you're sure that this looks good, click "Generate Gcode" in both windows.
ReplicatorG will generate the two different sets of Gcode and then put them together.
When it's done, you'll find yourself looking at that .gcode file you came up with a
name for earlier. Go ahead and print it just like you would any other generated Gcode!