I've been looking for something to help me out with a balance bot.. and I need gyro's to tell me not just 2 directions of movement, but all directions. My understanding is that 2 gyro's together can give me 360 degrees of sensing..
So if I am falling at 45 degrees(3 O' Clock) then I would have "falling" detected on both the 0 degree and 90 degree sensor.. and would simply move to balance that. As such, only two sensors are needed, one can tell 0 and 180, other is 90 and 270. The acceleramtors are to counter gyro drift.. so that if it is at an angel for more then a second, I can continue to have accurate readings, and counter its drift(which would tend to tell me its balancing itself out).
And yes, I am planning a ball balancing bot.. and yes, i do relize it will be a huge PITA and probaly not work but I feel its a great learning experience.