When you're doing the can-can on your BMX bike you gotta find a really nice jump that you're comfortable on. Now the jump, you should have a moderate amount of time in the air so that you can level out and what you're gonna do is, you'll have a foot that you're more comfortable taking off.
For me it's my right foot which is my dominant foot and my front foot. So what happens is my left foot is gonna drop, my right foot's gonna come up, go over the top tube of the bike and you're gonna just extend that leg out over to the opposite side. Once you've got your full extension you're gonna pop it back over the top tube of the bike. Make sure you clear the top tube, spot your pedal, get your foot back on the pedal, level out your cranks, squat your landing.
Once you're lined up with the transition you got both tires ready, you're gonna land both tires evenly. Again as you land you can always compress a little bit to absorb the landing and get like a nice pump at the bottom that will give you some speed but when you're doing the can-can that's really all you gotta worry about.
A little bit of hang time, pop your leg off, do your can-can, pop it back, get it back on the petal, level your feet, land both tires, ride away smooth. And that's how you do a can-can on your BMX.