Wondering

could we plug two dmg's in parrallel with the ps2 kbd??

wouldn't be polyphony but richer sound ...
has anyone tried ?

I'll have to do so then

If you are using a PS/2 keyboard plugged in to the link port, that will not work with mGB, only LSDJ. In which case it will not do polyphony.

If you are using a computer keyboard via the Arduinoboy, you have to have some kind of means of outputting MIDI with the computer keyboard.
If you are using a DAW that can use a keyboard to play notes, that must be enabled and you must be outputting to the midi output that is connected to the arduinoboy. Keyboard mode with the arduinoboy is not intended to automatically work with the computer keyboard, it is meant to emulate LSDJ's keyboard sync mode via MIDI.

You can read more about how the Arduinoboy works here: https://github.com/trash80/arduinoboy

I couldn't get it to work with the PC keyboard. Am I an idiot or does it not work or...

If you're using an arduinoboy, mGB will do polyphony. I don't think LSDJ does.

pretty much what the title says. Is there any way play the 3 gameboy channels with polyphony using an arduinoboy and a computer keyboard. LSDJ doesn't really work for what I want to do afaik.