Unfortunately, Utha, all I can do is tell you what comes to mind. I haven't the slightest clue how to go about doing it, though. Dardas says it's possible. I assumed most of it was possible, only thing I was skeptical about was the accuracy of timing, and the visual indicator.
I would say, a good start would be making the double attack feature work. The rest can fall into place after.
First off, a "Spell" for attack needs to be made. Based off the unit's Attack damage. (Basically, just a regular attack, but in spell form, since the regular attack is non-existant).
The timing is where I get stuck. I'd say, for the first one, 1 second after the first hit (So, "Unit Deals Damage"; exists? Perhaps "Unit Receives Damage", if not?) would be when the second hit would have to be. I'm not entirely sure how to go about doing this. My guess would be something along wait .9-ish seconds (.8 or .85 if .9 isn't long enough gap?) after that damage is done, then fire another trigger allowing the double attack to be run. Perhaps the need to utilize a Countdown Timer would be necessary, though, ofc., without a window. But I'm not sure if that's a good idea. Anyway, When the timer (Of second hit: 1s) expires, the trigger that's turned on just previously, will, upon expiration, turn on the "Double Hit" trigger, wait .2s, then turn it off. Why .2? Because .19s is the average reaction rate of a college-aged adult (Although mine is somewhere around 0.09-0.10 on average, which everyone is always amazed at).
So in this example, we have 3 Triggers:
Trigger 1: When the hero attacks, and the damage is received by the enemy, it fires off a 1s timer. Also, included in this trigger, is after .9-ish seconds, the second trigger will turn on. (I'm not certain this .9s is actually necessary, after thinking about it more. Leaving it for sake of completeness.)
Trigger 2: Simply a bridge. Event: Timer Expiration. Upon the expiration of that timer, turn on the third trigger, and after .2s turn off the third trigger (And this one, if the timer in the first one is necessary)
Trigger 3: Is the second attack trigger. Enabling that, upon the button press, the second attack will be run.
The ONLY issue I'm having with this idea, is where to put the "If Not Pressed, End Sequence" part. I'm guessing in the second trigger, after turning off the third trigger. There may be a check or something? I'm not actually sure.
I hope this was enough brainstorming to get you started. Cause I'm about all petered out here. If you know how to go about doing what I said, I really think that would be a good place to start. I have absolutely no idea how to do most of that, or if it's possible in GUI, or at all.
Good luck, hope to hear some positive feedback,
