In axis and allies, how are casualties allocated when two allies both defend a single zone?
According to page 19 of the rulebook at https://www.wizards.com/avalonhill/rules/axis.pdf

Defending together - when a multiple force is attacked, the attacker fires first as usual.  If a hit is scored, the defenders mutually agree which unit is chosen as the casualty (if they cannot agree, the attacker chooses).  When counterattacking, each defender rolls separately for his or her units (including casualties).