The rule states that reds are kos for opening or entering secrets. Most secret on most maps are breakables and this leads people to believe that any breakable that exists on the map is a secret.
The wall that divides the two cells is breakable, but it doesn't lead the red to any restricted areas nor does it reward them in anyway for breaking it. I think that breakable can be more properly defined as an "easter egg" that just connects two cells.
So no, you would not be kos for using that passageway or breaking it.