There is no A vs B button on the C64. There was only 1 supported joystick button, like the Atari 2600.*
Joysticks of this style had two buttons which both triggered the same functionality. I think it's only the 4 smaller buttons near the base of the Mini's joystick that map to additional keyboard presses per game, but there's very few actual official technical details released to answer such questions definitively.
- = Technically you could put additional buttons the analog paddle lines, but that was extremely rare in terms of software support. Some people also wired a 2nd button to trigger the 'Up' joystick direction, which is more comfortable for jumping in platformers.