We had a hard time for merging our custom plugin with Unity google cardboard SDK. Unity only provides single main activity in Android side. We used two tricks for using other activity in Unity main activity : using Intent android native function and Inheritance from google cardboard unity activity. By using these method, we could combine Google drive SDK, Dropbox SDK and Google cardboard SDK together.
The first method is using intent function in android native side.
http://forum.unity3d.com/threads/android-plugin-multiple-android-activity-problems.149089/
The second is Inheritance from google cardboard unity activity which is main activity. If tried to use this method, you need to make sure to overwrite all parent basic function by using SUPER class.
