How to alternate between two video clips in Premiere Pro CS6
You can use the multi-camera feature in Premier to achieve this.

From Adobe's page:


  To easily synchronize footage from all cameras, make sure each camera
  records a sync point using a clapper slate or other technique. Keep
  each camera recording to maintain synchronization. After you capture
  the footage in Premiere Pro, use the following workflow to edit the
  footage:
  
  
  Add clips from multiple cameras to a sequence.
  
  
  Stack the clips from each camera on separate tracks of a sequence.
  (See Add clips for multi-camera editing.)
  
  
  Synchronize the clips in the sequence.
  
  
  Mark the sync point with numbered clip markers, or reassign the sync
  point for each camera to a specific timecode. (See Synchronize clips.)
  
  
  Create the multi-camera target sequence.
  
  
  The final edits are made in a target sequence. You create the target
  sequence by nesting the sequence of synchronized clips into a new
  sequence. Then you enable the clip in the target sequence for
  multi-camera editing. (See Create a multi-camera target sequence.)
  
  
  Record the multi-camera edits.
  
  
  In the Multi-Camera Monitor, you can view the footage of all four
  cameras simultaneously and switch between cameras to choose footage
  for the final sequence. (See Record multi-camera edits.)
  
  
  Adjust and refine edits.
  
  
  You can rerecord the final sequence and substitute clips with footage
  from one of the other cameras. You can also edit the sequence like any
  other sequence—using the standard editing tools and techniques, adding
  effects, or compositing using multiple tracks. (See Rerecord
  multi-camera edits and Adjust multi-camera edits in a Timeline panel.)


If you like it more visually explained, take a look at this video: