well we tried and i saw the problem. you buffer a running Usmash instead of dash attack>usmash. the reason this happens is because you don't LET THE CONTROL STICK RETURN TO NEUTRAL POSITION.
if you buffer the dash by HOLDING the control stick and press up on the Dpad twice, you will do a running usmash and the second usmash you buffered will be totally useless.
If you buffer the dash by quickly flicking the control stick and letting it return to neutral position, you will still dash normally. THEN you input the two Usmashes. Since the control stick is in neutral, the first "Usmash" will come out as a dash attack and if you're fast enough, the second will cancel into an Usmash.
Timing the dash is key. Try buffering a dash out of a dthrow. then a dash attack, then a dacus (hence Buffered dash attack canceled up smash)
Good luck [;