Mutual attacks - where do they fight?

[Jasper]

This works fine if you're attacking from a province that is not being attacked in turn. If you launch from several provinces, including one that is being attacked, the enemy will attack first.

This has not been my experience. I have indeed launched attacks from several provinces and pushed back my opponent -- even though he was in turn invading one of the provinces from which I attacked.

Furthermore it's a logical paradox, as there is no way to differentiate "attacker" from "defender" when two armies in adjacent provinces try to swap provinces.

I'm curious why you are so certain it works as you describe? Did one of the developers say it worked that way?