If several units are fighting in the same combat, total up the number of Wounds inflicted by each side to determine who is the winner. All the units on the losing side have to check their morale. Individual winning units can only advance or consolidate if all of the units opposing them fall back.
Multiple Combats
Source: Core Rulebook 3E, p. 69
When several units fight in the same combat, total wounds inflicted by each side to find the winner; a winning unit can only advance or consolidate if every opposing unit falls back.