That's harder to pull off... I discovered the down-up-down input by doing a superjump motion and then letting go of the stick... my stick snaps back too far and registers an extra Down input after I let go.
Edit: To clarify, no, I haven't been able to pull that off yet. If it did work, it would probably require DF-UF-DF. If you start with a straight Down input, I don't think the air DF would count for a double-tap airdash.
I got it to work once with a DF airdash so far. Facing left, this is what showed up on input display when I got the DF airdash: 187414
I think the first 1 is buffered, the 8 does a straight up superjump, and the second 1 completes the diagonal airdash motion.
I'm not a fan of numpad notation but it's the easiest way to describe it in this case. Reference pic if you're unused to it like me:
Edit: Anyone know the timing window for a double-tap airdash? How many frames do you have after the first directional input to input the second one?
I just managed DF-UB-DF with no other directions inbetween and it did not work. It's possible I input it too slow.