If a person is pushed to their maximum, wouldn't they have discovered their branch power? 

Wait, I just a sweet idea. Scrap the word "power." The people are called Shifters, so let's call the abilities "Shifts." A shift in fire, water, visibility, strength, speed, whatever. You have the ability to shift a fragment of reality into the supernatural. This is called your Shift. 

All Shifts have an alternate extension of themselves. It is discovered upon extensive use of the main shift, through concentration/meditation or through adrenaline. This is called your "Alternate Shift," as it is a new power than can be used as easily as the first. 

The process of discovery is called the "Bridge." It is coined from finding a connection, aka a bridge, between two separate powers. They have a similar factor or property, in nature, design, impact or toll on the body and mind. In order to unlock your Alternate Shift, you must locate the Bridge between the two. The term has become a highly confidential password among shifters, shared only with other shifters who show the potential and drive to discover it. It is often used in a more discreet manner as a form of communication between shifters. Phrases like "Have you seen the new bridge?" and "I saw (person''s name) on the bridge yesterday" are small talk among normal people. Only other shifters know that it actually means someone has unlocked their AS and that the speaker has met them.