The best it to have several sets of keys.
The jam is 2 buttons in the way that only 2 buttons should be enough to play. But you can allow players to use several sets of keys, as they please.
In fact, it's better this way!
So you could give possibility to use:
- X and C
- K and L
- the 2 mouse buttons (I'm not a big fan personnally so in this example I would use X and C)
"Z and X" or "A and D" are a bit risky because for people who don't use a QWERTY keyboard they are not next to each other (I use an AZERTY of example).