Honestly, I got this by spamming dash, then spamming DD. Just make sure that after you dash, LET GO OF ALL INPUTS, and mash D.
Also, the timing of the attack isn't as important as the timing of the dash. The dash needs to be inputted as SOON as the 2nd 5DD connects. Keep in mind that the dash cancel is a COMMAND dash, so there will be a period of time where you can't do anything but dash even if you don't input anything, so it should give you a considerable amount of time to start mashing those DDs.
Try it out... Try doing 5DD and cancel it by doing a dash. He'll keep dashing even if you stop way before.