This is basically the same trick as the Backside Axle Stall; the only difference is that you carve into it with more speed so that rather than simply stalling with both trucks on top of the coping, you actually grind along before re-entering the ramp.
For a backside 5050 grind, it's pretty much the same technique as doing an axle stall, but you'll be going a bit faster and going at angle.
Use your momentum to grind yourself along the coping and back in, using the axle stall technique.
You'll come up, use your speed to get up on it at an angle, same thing. Lift your back trucks up, so your back truck is on, front truck on. Stand up, try and keep yourself on top of it over the coping. Grind along.
Lift your front truck off, weight on your back, pivot around. Bring it in the other way. You will probably be rolling away at an angle, not straight, so be ready.