Seems like this got a little bit under the radar, but there seems to be a commercial version of a dual feed, single nozzle extruder/hotend already that works pretty well: 3D Builder Dual Feed blog post

First GNU GPL public relase of FUM (fala union mono) update 11
scad file and stl file


link here

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[www.falla3d.com]
[3dita.blogspot.it]