@zaffy2005 A quick answer. So I'm not very confident about the accuracy, but if I was wrong, I know I can count on someone correct me
According to NA Arks Visiphone, it drops from Crimson Fellwyrm XH+ (https://pso2na.arks-visiphone.com/wiki/Twin_Machine_Guns_List). JP Swiki shows it drops from Black Fellwyrm too (https://pso2.swiki.jp/index.php?デュアルバード). If you want to be sure, you can check the quest log of these UQs.
Also JP Swiki mentions about Dual Bird dropping from Snow Banser/Banshee of level 81+ (but it seems that possibly it doesn't have enough reports to take the drop as a confirmed one). These level of Snows won't appear in regular quests. Some limited quests or seasonal UQs might have over 80 level of these enemies, if such quests come to the global, you can have a look at.
Synthesizers were mainly intended to work for the so called old type weapons. As we only have the new type weapons, their only use is to exchange them for pancakes at the pet lab.