Referencing second Visual Studio project sin Unity

Hi.
I am working in parallel on an Unity client and Photon server in two different Visual Studio projects. Unity project need to reference some classes form server project (eg. some enums identifying operations). Normally with two Visual Studio projects I would add a reference to the server project in a client project. But I cannot do this, because Unit is constantly rebuilding its own project files.

Anyone could suggest a workaround in this situation?

Hm, maybe you can help me to make a little work around - How in Unity can I add reference to a script outside of Asset folder? For example to some common libraries.