When 2 sideways-facing pistons are stacked on top of each other, and the top one is powered using a redstone dust signal, both pistons extend and retract.

I was able to bypass it by using a redstone torch placed very specifically, as can be seen in the attachment.

Apart from the above method, it's almost impossible to activate the top piston individually.

Powering the bottom piston using the same construction works as intended, with only the bottom piston activating.