This is fascinating when u dig in a little. It seams that Peter was paired with Andrew but also John, while John was in a pair with James. Thomas started with Mathew but finished with Philip. It appears that they were apart of different group at times.
The idea of pairs seems to be part of the Bible Moses and Arron, David and Nathan, Jeremiah and Baruch, Naomi Ruth, the two witnesses the list goes on and on. It seams to me that when were told that when two or three are gathered in my name, it is for much more than prayer. It seems to be how kingdom business get done.