2 high mobs won't get ejected to the right of the rail when in a 2 high room.
When looking at the attached screenshot (2016-11-20_19.10.15.png) the mobs ride towards the cactus. In that case other current Activator Rail behavior would suggest they should get dropped on the Spruce Block (where all passengers get ejected to when the glass blocks aren't there). But passengers get dropped on the Activator Rail.
Meaning a spot consisting of 2 air with a block above is wrongly rejected as a possible place to get ejected to.