3D and side-by-side contents are still scarce; the good news is, you still can watch any video with a Google Cardboard.
The first method is using Kodi. You need to set the pixel ratio to 2 and follow these steps:
Alternatively, you can install the SBSVideoPlayer on your phone. It is not a stand alone player though, it just converts a normal video to side-by-side within a video player. You open your video in a file browser such as ES File Explorer and select SBSVideoPlayer to open it:
The 2D video will look like this: