Best practice to make a transition between two displacement maps

Displacement values (scale, midlevel) are simple math operations.
Let disp(h,s,m) be the output of a displacement node receiving certain inputs for h height, s scale, and m midlevel.
disp(h,s,m) = disp((h-m)*s, 1, 0)

So you can do a little bit of math on the input to combine two displacements using different settings.