Microdisplacement problem I have 3 images for terrain - one main height map, one aplha selecting slopes (which I can replace with some nodes any time), and one height map of rocks for the slopes. Now I can use main height map for microdisplacement, no problem. But I want to have these details of rocks for the slopes too and everything rendered as microdisplacement. My PC won't handle traditionally displacing plane with main height map and then using microdisplacement for rocks only.

To clarify, I cannot mix both displacement textures with alpha fac because then rocks will only be elevated on z Axis, I want them to use normals of current mesh part they are displacing.

Here are main displacement and slope selection, I got rocks from cg textures so just imagine a simple rocks height map.
You can add the rocks hightmap to your Main hightmap, with a mix node (set to add, factor of 1), or a silmple math add node. Plug the output of the add node into the displacement socket. To control the strength of your displacement use a multiply node inbetween. (Values below 1 will reduce terrain hight, values above 1 will grow it.)
The slope information is only used for color, don't add that to the displacement. It will give you wrong results.