You don't mix them, but have them in your node network next to eachother, so one line of nodes go from the smoother terrain through compute terrain to surface shaders and into planet 1, the other line of nodes into planet 2. Only one planet needs to have an atmosphere of course.
The surfaces of both planets will intersect, so it needs some figuring out which one should be visible at which places. For instance, if both starting displacement fractals are the same at the beginning, but in one of them you slide the contrast a little to the left, you can imagine the tops going down, and the lower altitudes going up. With higher contrast the opposite will happen. You can also alter the the displacement 'wildness', and lots of other things.
This was made that way.