The arms on EV-9D9 are notoriously flimsy and prone to snapping off, as you have clearly already discovered :-(. I wish there was some way to do what you want, but I'd be amazed if there was. You could try the boil and pop technique, but sadly all I think is likely to happen is that the arms would snap off leaving the shoulder pegs inside the body.
If there was some way to prise the body apart without damaging it, then the arms could be removed without snapping them, but I am unaware of any such technique. If you didn't mind gluing the body back together again, the only method I can think of would be to very carefully saw through the front of the torso of one of the figures, and carefully remove all parts of the front with pliers, and then the repeat the feat cutting through the back of the torso of the other figure. You could then retrieve both arms, and then glue to two undamaged torso halves together again enclosing both arms. In theory that should work, and you would be making one whole figure from two damaged ones, but it would certainly be a fiddly enterprise, and you would always know it was a 'cut and shut' figure at the end of the day. Quite frankly, unless moving the figure's arms is crucial to you, it would be far easier and probably look better if you simply pulled the arm from one of the figures, accepting that it will snap off inside the body at the shoulder peg, and then simply glue it onto the other figure in the desired position.
Best of luck whatever you decide to do
