Make a collision sphere that is bigger. So you have 1 collision sphere just around the player for collision. You create a 2nd one that is 2 meters away. Then you check to see if player 2 has entered the 2nd sphere of player 1. Make sense?
So basically, you create multiple collision spheres on a character and when something enter the biggest one, you have the other player do something. So when a enemy enters the outer collision spehere, you tell that enemy to go somewhere else.
Here is a link to a video series going into depth about this. It is in javascript, but you will get a good understanding of colliders on a player to do stuff.
http://www.walkerboystudio.com/html/unity_training___free__.html