Not only does defender get to "attack" twice but the combat ratio will be more beneficial, since the attacking forces are counted separately. That significantly affects the math under the hood.
Example: Faction A has strength 3, Faction B has strength 1 and Faction C has strength 2.
If A and B attack C, you would expect the combat ratio to be 4vs2 but it's actually 3vs2 and 1vs2, separately. Which means that Faction will eventually win over Faction C, but not before Faction B has been grinded to nothing. And to make things worse, the attack by Faction B will most likely not cause much - if any - damage to Faction C because the combat is so against them.