Cant link PLUGIN.DLL with PLUGIN-EDITOR.DLL

I need help: I DONT FIND the proper way to split an existing working plugin int 2 dlls, can anyone help me?

I am spliting the original project into 2 projects: (runtime & editor)
- **the runtime does not contains any editor-related code**, and it creates the RUNTIME.DLL
- **the editor project** does not contains the runtime classes but it **includes the RUNTIME.DLL reference and only contains all the Inspector files**


I build runtime, then the editor.
It compiles, on Unity it recognizes the runtime scripts, and it recognizes some [MenuItems("tool")] from the editor.dll but it doesnt actually links both plugins...

Everytime I select an object, it shows an error about not finding the correct inspector **
* ERROR: Failed to load CustomEditor inspected type
UnityEditor.AssetPreviewUpdater:CreatePreviewForAsset(Object, Object[], String)
LOG: Can't load custom inspector theObjectInspector because the inspected type is null.
UnityEditor.AssetPreviewUpdater:CreatePreviewForAsset(Object, Object[], String)* **

This is how I place the dlls in Unity:.

Assets
. . .└ Plugins
. . . . . .└ plugin.DLL (the runtime)
. . . . . .└ Editor
. . . . . . . . . .└ plugin.DLL (the editor)


Am I setting wrong the VS project?
Am I setting wrong the Unity settings?
Why both dlls works but they cant find each other?
I already searched in every thread ( [this is the most similar post][1] , I tried, but still no fix for me)

Thanks a lot for your help! [1]: https://answers.unity.com/questions/1431766/error-cant-load-custom-inspector-because-the-inspe.html