After clicking on a BF key, you can click the secondary skill icon by either character's portrait, then click on the 'enter battlefield' icon.
Having one character used a ranged attack can help against mages that keep running or teleporting away. If you leave one character a distance away, then approach with the other, you should be able to get 1 or 2 mages to chase you back to where the other is waiting, where they will be short on stamina and unable to run away as much.
