can you do single backies in your sleep? where you'd do it off absolutely anything, big or small, because popping, setting the flip, spotting the landing, and slowing down or speeding up the last half of the flip accordingly is just second nature. that's where i think you should be before trying a dub because yes you just need to flip faster and tuck more on a dub, but if you are going to be in control of the trick you also need to spot the landing quickly as you come out of your first flip and act accordingly as you go into your second flip
a dub back feels like exactly what it is, two backies in a row, but you don't have quite as much control over the second flip because you aren't pushing off solid ground to set that one, you're just managing the flipping momentum you started 1 flip ago haha
so what i did and what i think you should do is get so comfy with singles that it's a joke and do em off big stuff of course, but then also do a couple off of something tiny so you get back used to setting and finishing a really quick flip because when you do singles a lot off big stuff you barely have to huck it at all so you get used to barely setting the flip. then when you know you're ready to commit, pick the biggest drop you feel totally comfy on and set it like youre doing a faster one, spot the landing almost immediately but don't un-tuck very much (if you're on a big ol' drop,) or at all (if you're on a small to medium drop), let your hips keep speeding around right into that second flip. THEN, you're back to your comfort zone where you're just spotting the landing (again) nice and early and adjusting the end of your flip accordingly.
basically, in my eyes the dub is great because most of its mechanics are ones you know super well from single backies. the tough part is getting your body out of the habit of spotting that landing and opening up cause that's what you're used to on singles. spotting the landing but continuing through that first flip and a quarter is the danger zone here imo. past that and all you're doing is the same flip you've always done. you know from singles that it's all in the hips, so don't let them slow you all down 1/3 the way through your trick, but DO spot that landing so you know what the hell is going on