The Merge Shader handles both Colour Input and Displacement Input (which either can be toggled), and acts about the same. I believe the displacement uses Vector displacement, but I may be wrong, but have had success merging lateral displaced terrain, so, but Matt also specifically may have developed this plugin to handle his terrain types more appropriately.
Additionally, you can convert your terrain(s) with Displacement to Scalar and manipulate it with functions, including mixing this way with the Add Scalar, Multiply Scalar, Divide Scalar, Mix Scalar, Subtract Scalar, Difference Scalar, etc.
You can also work with Convert Displacement to Vector and the Red to Scalar (X), Green to Scalar (Y), and Blue to Scalar (Z). You can than edit each respective scalar with shaders I mentioned above, and than rebuild the vector with Build Vector