I want to join the rock and the body but keep the UVs.
I renamed every UV Map but if I join them you can see in the box that both UVs are together but I can only enable 1 UV for rendering and the other one hides automatically. How can I fix it? :((
You have 2 options: you can assign different materials to your selected faces OR you can make an UV atlas.
The general workflow for the UV Atlas is that you have to contain both of your maps in a single UV space. Since you have 2 UV maps, make sure you take apart UV map 2, and then merge your mesh, so both are not overlapping. Then you fix it into a selected area within UV map 1 like this picture shows here.
If you already painted textures, and do not want to do Uv mapping, you create a vertex group of your UV map 2. Merge both meshes. Create a new material to assign to the vertex group of the UV map 2, and assign it in EDIT mode.
Your shader should point to texture corresponding to UV map 2.
There are quicker options, but I don’t know your level of expertise in Blender. I hope these two methods could help you lighten this issue you’re having.