Steps to repro:

1. Open Unity 5.5;
2. Create new project;
3. Extract attached 'ShadowsCullingBug' package;
4. In the project folder go to Assets -> SampleScenes -> Scenes;
5. Open 'Car' scene;
6. Launch the scene;
7. Switch to 'Free Look Camera Rig Mode' by clicking on corresponding UI menu in the top left corner of the Game view;
8. Ride the car model to the left side of the track to enter into the shadowy zone;
9. When being in the shadow zone rotate the viewpoint to see the car's front side;
10. Observe shadow rendering.

Actual result:
Shadow culling produce artifacts when dynamic shadows overlap each other. See attached gif video and actual.png screenshot.
Regression introduced in Unity 5.5.

Expected result:
Shadows should be rendered without artifacts when 2 shadows overlap each other (see attached expected.png screenshot).

Notes:
- Reproducible in 5.5.0b2;
- Not reproducible in 5.4.0f3, 5.4.0p3;
- Reproduced on both OSX 10.11.6 and Windows 10;
- Reproducible in both Scene and Game views;
- Reproducible in both Editor and Standalone build;
- Reproducible for both realtime and baked GI.