If you place three pistons in a row (all facing up) and place slime blocks on top of them, when both the left and right pistons are powered, one of the pistons stays on the ground and the other two (along with the slime blocks) are lifted up in the air. It seems that which piston stays on the ground should depend on where the power is placed, and this is true. However, the piston on the right seems to stay on the ground more often than the left. This means that 2/3 time you place the button in a random spot, the right one will get powered.