I have to say... the easiest solution in my opinion is the $10 mouse hack from Oscar. I tried hacking one myself (that I bought for 4bucks) and creamed it... For 10bucks and its done..
NOTE: He only usually has the ground for one axis (since he uses it for spinners)... I don't remember what he charges for a second wiring harness... but it was also super cheap.
Next, you have to combine the grounds. Takes about a second to do.