The original hack was made by Ryan (abslodoodle37) and here's some documentation on how to do it yourself.

First of all this requires some soldering skills. If you thought that attaching one motor was a lot of work, it's tricky to attach two motors into the same holes.

Aside from a Crazyflie you will need 4 spare motors and 4 spare propellers (2xCW and 2xCCW). If you are starting from a fresh kit then you will at least need three extra spare motors. So:

• 8 motors
• 8 propellers (4xCW and 4xCCW)
• 4 normal motor mounts

1) Remove the protective edge bottom of the motor mount that stops the motor from sliding out at the bottom

2) Twist the wires for the two motors and insert them into the mount. You can either do like the image with each motor twisted or twisting black and white together from the two motors

3) Mount the motors in the holes. Follow the normal mapping for the top motor and inverse it for the newly added bottom motor. So where ever the black wire is attached from the top motor, the white should be attached from the bottom motor and vice versa.

4) Now mount the propellers. Follow the normal mapping for the top motors and the same mapping for the bottom motors. So if a CCW propeller is attached on the top motor, a CCW propeller should be attached on the bottom motor. Both the propellers should the facing the same way, so don't turn the bottom propeller up side down.

5) Attach some landing gear or find something like a CD-spool to launch from

6) Done! Here's a video. http://youtu.be/Llj7umnwbH4?list=UUTVODP4fWj_ZuwvRyhhXd7A