I made a sync device that will synchronise up to two copies of Nanoloop 2 to a MIDI clock source. The device is powered from on of the Game Boy Advance consoles, via swapping pins in order to gain access to a 3.3V signal.
The device also supports a "duplet / triplet" type mode. By sending a CC value to the device on CC number 1, the tempo relationship between the host and the slaves can be changed. Channel 1 is GBA console 1, and channel 2 is GBA console 2.
CC#1 (Modulation): 0 <= 31: slave plays 2 notes for every 2 notes that the master plays
CC#1 (Modulation): 32 <= 63: slave plays 2 notes for every 3 notes that the master plays
CC#1 (Modulation): 64 <= 95: slave plays 2 notes for every 4 notes that the master plays
CC#1 (Modulation): 96 <= 127: slave plays 2 notes for every 5 notes that the master plays.
Each slave GBA can be set independently. By combining the above rhythmic ratios, interesting rhythms and grooves can be established. See this video for a brief demo: http://www.youtube.com/watch?v=HeW_zMVoWts
Download the Arduino Code here:
The schematic is very simple, it's basically a simple MIDI in circuit, with VCC coming from the Game Boy Advance #1. Digital pin 7 of the Arduino goes to Serial In on Game Boy Advance #1, and pin 8 goes to Serial In on Game Boy Advance #2. Add 0.1uF and 47uF caps between ground and VCC.