What is the difference between a 'live' NLG axle and a 'fixed' NLG axle? What are the advantages / disadvantages of a live and fixed nose landing gear axle and how do they work? Maybe certain aircraft need a particular type fitted? 
I can't see the need of a moving axle (assuming that's what 'live' means) unless it is somehow powered by a motor or something similar for push back..

Or.. 
Maybe on an aircraft a live axle is one where it rotates with the wheel and a fixed one is where the wheel rotates around the axle? 

The reason I ask is that I saw in a textbook that nose wheels are either on a fixed or live axle and nose wheel shimmy is prevented by using a twin nose wheel set up attached to a live axle.

I'm now not sure if the terms live and fixed axle are in relation to the vertical component or horizontal component of the nose gear.
A live axle is when the two wheels rotate together. Its function is to reduce shimmying.

The Lockheed JetStar and the Space Shuttle Orbiter have live NLG axles.



References:


Source: Aircraft Landing Gear Design: Principles and Practices


  The nose wheel is a split, forged aluminum alloy design but its bearings are located on a rotating or "live" axle rather than in the hub such as in the main wheel design. Both nose wheels are splined to the "live" nose axle providing a corotating feature which improves stability or reduces the tendency to shimmy.


Source: Orbiter wheel and tire certification, NASA


  Tire friction can also be used to increase the damping in the castor mode. If the gear has two wheels, connecting them by a live axle can have this effect. In the case of a single wheel, it is possible to obtain ‘anti-shimmy’ tires incorporating two contact areas.


Source: Landing Gear (Aircraft), sciencedirect.com