You'd have to set up events to see if the down button is pushed twice within a certain timeframe.
I guess the easiest way would be to create a variable and on down pressed add 1 and set a timer behaviour to the amount of time available for the second push.
on timer end set variable to 0
on down pressed
is jumping
if variable is 1 set normal attack
on down pressed
is jumping
special attack is not active
if variable > 1 set special attack