code: function(tr1, tr2, doCreate){
  
              var hash = pairHash( tr1.id, tr2.id );
  
              if ( hash === false ){
                  return null;
              }
  
              var c = this.pairs[ hash ];
  
              if ( !c ){
  
                  if ( !doCreate ){
                      return null;
                  }
  
                  c = this.pairs[ hash ] = {
                      bodyA: tr1.body,
                      bodyB: tr2.body,
                      flag: 1
                  };
              }
  
              if ( doCreate){
                  c.flag = 1;
              }
  
              return c;
          }
docs: internal
  SweepPruneBehavior#getPair( tr1, tr2, doCreate ) -> Object
  - tr1 (Object): First tracker
  - tr2 (Object): Second tracker
  - doCreate (Boolean): Create if not found
  + (Object): Pair object or null if not found
  
  Get a pair object for the tracker objects