Probabilistic cloning with supplementary information contained in the   quantum states of two auxiliary systems
  In probabilistic cloning with two auxiliary systems, we consider and compare three different protocols for the success probabilities of cloning. We show that, in certain circumstances, it may increase the success probability to add an auxiliary system to the probabilistic cloning machine having one auxiliary system, but we always can find another cloning machine with one auxiliary system having the same success probability as that with two auxiliary systems. 
Lvjun Li, Daowen Qiu