suka made a split keyboard that connects the left and right switch matrices with wire and no I/O expander (instead of wireless like leeku or wired with I/O expander like ErogDox).
Here is a picture of it:
Show Image
(http://deskthority.net/resources/bluecube-inside-view/3109)
I think this would be the simplest way.
But suka's wire is rather short.
My wire would be 1.5 meters long.
Would 1.5-meter long wires have too much impedance for the little controller?
Have you ever seen left & right keyboards connected like this using a long wire?
suka's keyboard collection is posted on: http://deskthority.net/posting.php?mode=reply&f=7&t=2534&sid=634f6d6d8f07e65e4c9e576762ad8d11
wire will be "fine"... I don't see a problem.. at worst it doesn't register, when you press down too many keys.