Summary

These are the parts you need to make a dual head extruder for your M2.
The very base core design of the extruder drive, spool holder, and end stop, files come from MakerGear with heavy modifications. I would like to thank MG for making these files available on their GitHub.

I had to completely redesign the motor mount and the print cooler is entirely original.

Look on the instruction tab for more notes on this build.

It is still a work in progress and suggestions are welcomed.

Instructions

These files are provided in the hopes that they will be helpful but are in no way a promise that they will work for you.
I make no claim that this procedure will not destroy things. I have not yet destroyed anything on my machine but your mileage may very and I will not be responsible for any damages. Have fun.

I have been working on this for the last month and may have forgotten a few parts. If you see something then please let me know.

You will need one more hot end, another extruder motor with wiring harness.
Wire to go from rambo up to new hot end and thermistor.
The wiring connectors to plug the new motor, hot end and thermistor into the rambo.
A bearing, M5x18mm cap screw with nut and M4X16mm with nut for the other side of the extruder head.
Two M4x24mm cap screw with nuts for motor mount clamp.
Two M3X20 with nuts and one M3x12 with nut to mount the 50mm bed fan and shroud.
PTFE guide tube to guide the filament from the new spool.
Zip ties to attach the wire harness to the motor mount.
A sense of adventure.

The dual spool holder is made of three files. 2 files are clips that help support the weight of the spool but so far I have not needed them. The first version I printed did not have long enough posts to hold my spools and so I made the posts longer but have not printed it yet.
It uses the same hardware as the original spool holder.

The motor mount uses the original hardware to mount on the machine.
Install one motor and then the drive head and then the other motor.
keep the screws lose until you get all of them in each motor. When you tighten them all down make sure you are not binding up anywhere.
The stepper motor drives are at a 45 degree angle so that there is a corner on top and there are only 2 screws in each one holding the drive head on.

You will need to print the extruder drive head standing up in the same orientation it is installed in. I used some support in the hot end slot but did not need it anywhere else. I had problems with the filament hole being way to small and increased it a bunch but it was still tight so I ran a drill bit down it from the top and bottom (before installing it on the motor of course.)
There can be a bit of clean up needed in the hot end slots and especially on the right side where you want to be able to move the hot end up and down a bit. The clamp for the hot end uses two M4x16mm cap screws and lock nuts. ( I think they were 16mm long but may have been 18mm)

The print bed fan is designed for the original ceramic core heater hot end. I don't have the new version so I do not know if it will fit. In fact I am not sure the new hot ends will fit in the dual extruder head. If someone would like to send me a couple I can redesign the fan shroud otherwise it will have to wait until I need to get a new hot end.
There is one M3 cap screw holding the shroud to the extruder head. I think it was about 12mm long.

The new x end stop is needed because otherwise when you home the machine it will crash the hot end into the side. It is a bit close with this new one and I may make it longer at some point. It uses the same hardware as the original.

After you assemble everything leave the hot end clamp a bit lose so you can move the hot end up and down. Creep the bed up until it just touches the fixed hot end. With the lose hot end touching the bed tighten the clamp. It would probably be easier to do this with out the bed fan in place.

When you test the hot ends watch the temperature closely to make sure you have the correct thermistor on the correct hot end. If you start it up and the temperature does not increase check the hot end and if hot then you have the thermistors reversed. Quickly shut it all down and switch them on the rambo.

S3DCreator does not have the ability to set an offset for the dual extruder, relying on the hardware to know where the other hot end is.
I understand that Slic3r does but I did not try it.
I think you should have and firmware take care of that end which will make our gcode more universal and allows you to reuse the gcode later even if your offset is a bit different.

The original M2 firmware does not have the functionality to set an offset so if you are going to use S3DCreator you will need to get the latest version of Marlin and modify it to work with the M2. There are several versions of the M2 out there now with different voltage power supplies and 2 distinct rambo board versions that makes a one sizes fits all solution very difficult.
I have the firmware that works for me on my GitHub. https://github.com/ErikAkia/Marlin I have the original machine with 19V/12V power supplies and the rambo that has 8 micro steps.
I based my changes on Jin's GitHub here https://github.com/jinschoi/Marlin/tree/MakerGearM2 I know he is running a 24V, 16 microstep with v3b hot end. You will need to modify his files for the dual extrusion if you have the newer M2.

I am going to work on a nice calibration object and will include it later. Essentially you need to figure out approximately the offset between your two hot ends and then make the change in the firmware. Print out a test and then adjust. It is a very clunky system and needs to be updated. I would to be able to set the offset in the eprom from a M code at the very least.

Some random thoughts
The hot end that is not printing oozes everywhere so it needs to have an extra bit of extrusion when switching tools to it and then retracted when switching away. Think about how long a hot end will not be used and think about adding another part to the bed that can be printed to with that hot end to keep it fresh. If you purge your hot end before starting then you need to do both. The skirt does not work very well with both hot ends so a new start gcode should take care of that function.

Dual Extruder for M2