If you're interested really specifically in how to shoot doubles on southpaws, the best people to watch are probably GSP and Khabib, though they do it very differently. GSP will shoot reactive takedowns on lefties that are less doubles than they are kind of knee blocks, he'll pick the right leg up, block the left knee with his hand, and run the guy over that blocking hand. Khabib will just shoot a southpaw double out of an orthodox stance (I do this too and find it works pretty well). So he'll throw a few punches to get the guy to shell up and then take a small step forward with his leading left foot and shoot in hard on the hips with his head still on the left side. What I like about doing it this way is that for me at least most of my doubles come off of jabs or jab feints since those get my head moving in the correct direction, whereas doing it this way comes off of a cross or cross feint so it's a bit of a different look. I especially like to set it up with a few big overhands to really get my opponent bringing their hands up when they see my dip, you can get some really easy takedowns off that reaction.