An user posted a project to the Unity forum where a main camera renders the skybox and some white cubes, while a send camera renders a green cube in one eye and a third camera renders an orange cube in the other eye. I replaced the plugins folder in the user's project with my own plugins folder that contains the SIG files for my phones, and tried it out.

Using Unity 2017.1.0f3 I saw the problem occur on S7 Gear VR with Mali chips and S8 Gear VR with Snapdragon chips. The skybox and white cubes appear in both eyes but neither the green cube nor the orange cube appear in either eye.

Using Unity 2017.2.0b8 the problem does not occur on S8 Gear VR with Snapdragon chips (did not try the S7). The green cube is in one eye, the orange cube is in the other eye, and both eyes show the white cubes and skybox.

Here is the forum link:
https://forum.unity3d.com/threads/target-eye-right-doesnt-work.456343/#post-3193683
11:58

The forum postings mention the following (case 861752):
https://issuetracker.unity3d.com/issues/vr-when-having-two-cameras-in-a-scene-only-the-right-cameras-view-is-rendered