By exploring the basic inequality, the explicit expression of the average failure probability of the probabilistic m→n quantum cloning of two quantum states with arbitrary a prior probability is given. Results show that when the number of copies approaches infinity, the probabilistic quantum cloning converges to the optimal unambiguous quantum state discrimination. The average failure probability depends on a prior probability, which makes the copies of one of two states can not be always obtained for some a prior probability. By enhancing the average failure probability, the copies of two states can be always obtained for arbitrary a prior probability.