So in this case the blit setup is faulty and leads to this behaviour.

To avoid this problem there are several things that need to be done:
1) Main culprit of the problem is the fact, that the blit samples CameraOpaqueTexture, which is not updated inbetween the blits and therefore the second result looks unaffected by the first blit as it blitted based on the color copy which was made before the user's renderer feature blits. The Sample code can be changed in the blit shader to use _BlitTexture but ONLY if user will do what's suggested next
2) Create a temporary RTHandle to handle the blit and target swap. The color copy in this case is not needed anymore, so the target count remains the same. But the main reason for this is, that you cannot sample the texture (in this case it's _BlitTexture - the source RT set by Blitter calls), which is currently the active render target. So the blits would look like
Blitter.BlitCameraTexture(cmd, m_CameraColorTarget, tempRT, leftMaterial, -1);
Blitter.BlitCameraTexture(cmd, tempRT, m_CameraColorTarget, rightMaterial, -1);