Hi Dave.
I use 1.2 mtr lenghts of decking varying from 1/2in to 1 inch in thickness to raise both wheels together on the side that needs to be raised,I carry 3 lenghts but only have ever used 2.Levelling front to rear is the same as a single axle,but more care should be taken when selecting a pitch as you have more of a overhang which makes it easier to touch ground.
I hope I have explained it so you can understand me
Allen