So far, there doesn't seem to be a truly effective method.
The event is tracked using trigger #16, after temporarily disabling the hero (in this case, all heroes) using SetObjectEnabled.
With the hero disabled, they don't have a "normal mount" (they can't interact with all objects themselves). Also, the hero will be perceived as an impassable object.
For example, when two heroes interact, the attack from the enemy hero won't be recognized as an event – on simultaneous turns.
Added 1 minute ago
Also, the coordinate method, as used in The Restoration of Erathia, is not suitable. Because it's not necessary for them to be in contact for the event to work.
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