I'd buffer by default and use a visual cue animation (or at least pose) to indicate when the player has a jump buffered. Perhaps by timing the button press closer to landing the second jump could be slightly bigger. Making the visual cue tell the player what the game is doing and introduce a neat skill into the gameplay.
I'd also add a press and hold timed second jump if the player presses the key before landing and holds the key until touch down then releases at the right time you could create a nice parcours jump that lands the character facing the other direction. So few games use the old 180 turn anymore and SO should.