I read into filament making, and from what I remember people where having the best results when they had a very exact rpm on the winding machine and were pulling the filament through water. Faster rpm= thinner filament, slow= thicker. Though you probably already know and do this..
I suppose a tube could work but how is it leaving the second tube any different from leaving the primary extruder? Would the filament be cooled already at a larger diameter then cold extruded through the tube, or still warm? An interesting project, if it works out well I may look into one of my own.
If you need some help, or don't understand what I just said, feel free to send me a PM anytime
Printer: Prusa i3, 2 E3D v6 Hotends, Arduino + RAMPS 1.4 with a Bypassed 5V Regulator, 400w Insignia ATX PSU, Custom Designed Bowden Extruders