your question was poorly worded.
The answer depends on how much leveling you need.
MOST tandum axles there can easily be 4-6 inches difference between the ground height on the two axles with no problems. So you can place blocks under just one AXLE unless you need to lift it several inches. I will describe some stair steps you could use at the end of this post.
HOWEVER if there is more than one tire on the wheel (LIke on my class A it has "Duals" on the rear,, two tires on each end of a single axle) You must suppoort BOTH Tires (The entire wheel)
so a tandem axle where one tire is in FRONT of the other, no problem
A dual axle where the two tires are on the same hub.. Big problem.
My stir steps are 2x wheel tire tread width
One foot
Two foot long
Three full feet long
Glued one atop the other.
For you it might work better if you went
Two feet
Four feet long
Six full feet long