Dual Wingbeat is better that Aerial Ace in almost every regard. For starters, since Dual Wingbeat hits two times with 40 damage per hit, Technician is applied to both hits, thus resulting in 1.5 x 40 = 60 damage per hit and 120 damage total. In comparison, Aerial Ace's 60 base power gets boosted to 1.5 x 60 = 90 base power, which is 25% weaker than Technician boosted Dual Wingbeat. Even if Technician isn't being used, Dual Wingbeat still has 80 base power total, compared to Aerial Ace's 60.
Additionally, Dual Wingbeat beats Focus Sash and Sturdy Pokemon, and since such tactics are commonly used to stop sweepers, having a move that effectively avoids it is great. Other niche interactions include hitting Multiscale Dragonite for slightly more damage, as the first hit breaks their ability while the second hit has full power (the same does NOT apply to Tera Shell Terapagos, as pointed out by AlolanRaichu.)
The ONLY notable advantage Aerial Ace is that it can never miss, so if you're playing a format with a lot of Pokemon spamming evasion moves, that could be useful. In comparison, Dual Wingbeat's 90% accuracy can be frustrating, but at Stephwheel mentioned, "git gud" (90% accuracy is usable already.) Aerial Ace also has double the PP compared to Dual Wingbeat at 32 maximum instead of 16, but that is a very minor difference and won't matter unless PP stalling is a huge concern of yours (which shouldn't be the case in many competitive formats.)
