I write this short note, an abstract from the - in-print next October - issue of The Shed Magazine. I should admit that I had never though to this important detail without spending a lot of time in chat with rachaelp trying to understand and solve the problems of her first "dual extruder" 3D printer. Sometime sharing our own issues maybe one of the best forms of cooperation and knowledge sharing.
A dual-filament extruder maybe a trap, so please be careful! Dual filament does not mean that the extruder can pull one or the other filament depending on the design, instead it means that two filament spools are involved at the same time. If you try to use a dual filament extruder (that has a single nozzle) with a single lament feed, it will not work. Use a dual filament extruder only at times you choose to play with mixing colours together or if you plan to consume a lot of material.