Saint Paul (on the right) was an apostle, but not one of the 12 apostles of Jesus. He became an apostle only after the Ascension of Jesus. He was known as the apostle of pagans, as he preached and spread Christianity from Spain to Arabic nations. He was martyred with Saint Peter (on the left) around 64 AD, during persecutions of Nero. 

Saint Peter (on the left) on the other hand was the leader of 12 apostles and as such appears often in the New Testament. In the icon, he can be seen carrying a key. This motif is very popular in icons depicting him. As saint Paul was known as the apostle of pagans, Peter was known as the apostle of the Jews, as he preached mainly to Jewish people. He was crucified upside down 67 AD and as such an upside-down cross is called The Cross of Saint Peter.