I haven't fitted Andreani cartridges in my fork, but I've read a lot about it and I've fitted Wilbers progressive springs in my forks, so I well know how the fork is built.
Fitting an Andreani cartridge in the right leg is quite easy as there is already a damping cartridge in and the Andreani cartridge comes in place of it.
In the left leg, it's lot more complex : you need two specific tools, a first one to remove the inner cap upon the spring, a second one to remove the fork tube from the bottom part of the fork. Then, you need to modify the bottom part of the fork, so that you can fit in the cartridge. Maybe, you can just dismount the fork by removing the tube from the bottom part, don't know if it's possible when the spring is still there compressed within the tube.
As far as I understand, you really need to be well equipped and skilled to modify the left leg.
Another solution is mounting a Maxton cartridge only in the right leg : less expensive, a lot more easy, but certainly a little bit less efficient