Displacements may sew with one another so long as their vertices meet - this means the two either being brushes of identical dimensions with identical displacement powers, or identical dimensions with different displacement powers (the vertices on one brush that don't have a corresponding one on the other can still sew to the edge of the other brush). Your biggest worry about vertices not being able to sew occurs when you start resizing other brushes...
Don't forget to align the textures on these displacements, as well. Sometimes when connecting these together and painting the alpha, one forgets that displacement textures still have to be aligned right, otherwise things will still look odd (such as the ground in that image posted).