I have one on my 99MM700. It is the Mountian Machines axle. It works fine. The thing I worry about is if a bearing in a wheel goes bad, your day is done where you are on the mountain unless you have another wheel along.
I am also running the two wheel kit on my TRX-1. I am going back to a three or four wheel setup for the above reason.
The two wheel kits do look cool though.LOL