If I understood correct, if you dash dance, you will have the standing animation for one frame when you dash from one direction, too the other. You want to overwrite the dash you input, with something else (attack or just standing in place). To do this, you simply need to make another action, before he goes out of that one frame window.
Empty pivots:
Flicking: When you dash one direction, you simply flick the stick fast the other direction and release it, so the stick JUST enters the dash sequence your looking for.
20GX way: You dash one direction and then dash the other...but aim for the notch that is between crouch and dash. I believe Luigi crouch if you aim for it, so you need to be right above it, and then push it down real quick. I personally think this is the best one, but feel free to find what works for you.
Regular Pivots: This is simply done by example, dash one direction and try do Fsmash in that 1 frame window the other. This will look like Luigi don't finish his dash animation, but stops and Fsmash mid animation. If you miss it, you'll probably will dash attack, or just finish your dashanimation.
BluEG playing around with emptypivot:
https://www.youtube.com/watch?v=jYMpSZdkqCc&feature=youtu.be
Gravy explains how he does his pivots:
https://www.youtube.com/watch?v=MtRPjZ37cO0
PewpewU show how he uses regular pivots:
https://www.youtube.com/watch?v=J_aqH9sJnbU
Hope this helped. Btw, even tho you probably read that "empty pivot is the new tech that Luigi GOT too learn", it's a frickin hard tech. I can't do it more than 2/10 consistently, and cannot rely on it in matches. It's no shame in starting with WD down to stop your momentum, until you have learned too do it atleast 9/10