Hi,
We are going to add some errata to clarify things.
TW pg 175 - COMBAT WITHIN BUILDINGS
"Unlike standard combat, units in the same building hex (and even at the same level in the same hex) can attack one another."
CHANGE TO:
"Unlike standard combat, units in the same building hex (and even at the same level in the same hex) can attack one another, treating the target as if it was in the Front Arc."