Steps to reproduce:
1. Download and open the attached project
2. Enter Play mode

Expected result: both eyes receive the effect and can view the full viewport
Actual result: viewport gets reduced for the right eye

Reproduced 5.4.5f1 5.5.3p1*, 5.6.0f3, 5.6.1p1, 2017.1.0b5
5.5- results in the whole view being distorted

After discussion with Sergio Gomez confirmed this has been fixed in 5.6.3p2 and above, with the change to the user script outlined by Dylan Fitterer in the parent case. To be clear, in the user project, in MultipassImageEffect.cs, in method OnRenderImage change the 'var temp = RenderTexture.GetTemporary...' line to use VRTextureUsage.TwoEyes as follows:

var temp = RenderTexture.GetTemporary(tw, th, 0, rtFormat, RenderTextureReadWrite.Default, 1, VRTextureUsage.TwoEyes);
Confirmed fixed in 5.6.3p4 (latest 5.6/staging) with that change.