If you have room, then I would add two of them, just jam one to the left of the player one stick, and one to the left of the player two stick. You can install them with the bases nearly touching the 8-way bases. There are not a lot of two player simultaneous 4-way games, but there are a few, and also a few dualjoy 4-way games as well. The Ms. Pac reunion stick is also excellent for puzzle games, many of which are two player.
If you don't have room for two, then either center it, or stick it next to the player one stick.