Regional "Alliance/Hostility" changes

[Malice-]

Tried a search about this, got nothing.

Say I have a map with two separate zones and a small open gate area that connects them. Say the top zone is just for training your hero against NPC creeps, so you can't fight other players up there. And say the bottom zone is an arena-like area where everything should be hostile towards you(except your ally if you have one). How would I go about making a trigger that changes that between the two zones? In the top zone, no players should be able to attack each other. And in the bottom zone, everyone should be hostile towards you.

Any ideas?

 

[ScreWe]

Untitled Trigger 001
    Events
    Conditions
    Actions
        If (All Conditions are True) then do (Then Actions) else do (Else Actions)
            If - Conditions
                ((Playable map area) contains (Triggering unit)) Equal to True
            Then - Actions
                Player - Make Player 1 (Red) treat Player 2 (Blue) as an Ally with shared vision
                (REPEAT FOR ALL PLAYERS)
            Else - Actions

Do this for both regions.. make one be ally one be unally