MM is not a two-button mouse. There is a capacitive sensor under the front of the mouse that simple determines if there is a single finger over the right side of the button. If there is, it's a right click. Otherwise a left. There can be no "both."
Obviously if you use your left finger you get a left click.
If you click the button by pressing the mouse from the rear, you get a left click.
If you use two fingers you get a left click.
Only if there is a finger over the right side of the front will you get a right-click.
So logically, it's not an A+B but an A or B function.
As the other poster noted, you can program the scroll button for whatever your game requires as a left-right click. Or you can use either of the side buttons.
Or you can get a Logitech mouse with two real buttons for left and right.
