Try this:
(Event)
Unit - Unit Enters a Region
| . REGION GOES HERE
(Condition)
Integer Comparison
| . Unit - Count Units in Unit Group
| . | . Units In Region Matching Condition
| . | . | . REGION GOES HERE
| . | . | . Boolean Comparison
| . | . | . | . Player - Player Is An Ally Of Player
| . | . | . | . | . Owner of Unit
| . | . | . | . | . | . Event Response - Entering Unit
| . | . | . | . | . Player 1 (Red)
| . | . | . | . Equal To
| . | . | . | . True
| . Equal To
| . Unit - Count Units in Unit Group
| . | . Units In Region Matching Condition
| . | . | . Playable Map Area
| . | . | . Boolean Comparison
| . | . | . | . Player - Player Is An Ally Of Player
| . | . | . | . | . Owner of Unit
| . | . | . | . | . | . Event Response - Entering Unit
| . | . | . | . | . Player 1 (Red)
| . | . | . | . Equal To
| . | . | . | . True
Basically that checks, when a unit enters the region, if the number of units in the region allied to Red is the same as the number of units anywhere allied to Red, and if so, executes the actions.