In the canonical novel Journey to Star Wars: The Force Awakens: The Weapon of a Jedi - A Luke Skywalker Adventure, Luke recalls the incident in the Mos Eisley Cantina as follows [emphasis mine]:
Luke’s mind flashed back to the Mos Eisley cantina, where two alien thugs had picked a fight with him. Ben had tried to play peacemaker, sensing Luke’s growing panic, but the aliens hadn’t been interested in peace. One had flung Luke into a table, then gone for his blaster, ready to gun Ben down.
The old Jedi’s hand had dipped to the lightsaber on his belt, faster than anyone would have imagined a desert hermit could move. His lightsaber sliced the blaster aimed at him in two, then carved through the thugs.
This makes it clear that Obi-Wan delivered two strokes, and that the first had cut the alien's blaster in half; only after this did Obi-Wan chop off his arm.
The question, then, is simple: