Anet A8 Dual Extruder

A mount for two Ender 3 Extruders on a stock Anet A8 (with a X-tensioner on the back).

## V0:
Things i changed / fixed:
- original carriage did not hit the x-endstop.
- in the original carriage the bearings where 1mm too close apart for my printer, which lead to the carriage being unmovable (you might check this for your printer with the carriage_test.stl first)
- alternative boltholes between carriage and extruders.
- the pins on the top where intended for an easy to swap motor-mount (f