Include two of the same DLLs in one solution and Visual Studio

I would like to include two of the same DLLs (different versions) in my project. One is used by one of the plugins (can't be moved or removed) and the other one is used in the project itself (can be placed anywhere BUT the plugin folder which seem inaccessible from the scripts). So technically I need two of the same DLL in my project.

This works fine in Unity/Monodevelop but Visual Studio won't build the project (the same existing classes/namespace conflict with each other).

Solutions:

1. extern alias doesn't seem work properly in Mono/Unity? -> internal compiler error

2. Editing DLL properties in Visual Studio (Alias, Specific Version) works -> but is overwritten later when Unity overwrites the csproj files

3. Editing csproj and adding the SpecificVersion element dynamically in a Unity PostProcessor script failed -> as Visual Studio was losing the reference to the DLL for some reason (the csproj file looked fine, no idea what caused this)

Did anyone deal with a similar problem? How did you solve this?