First things first, you should NOT be housing two snakes together in the same enclosure. You're asking for a long list of problems by keeping them together, but you can search the forum for threads on cohabitation and red up on it there.

Second, get rid of the heat rock. Heat rocks should never be used with ball pythons as they get WAY too hot and can burn your snakes.

Third, the issues you're having with the shed is due to low humidity. From your picture, it looks like you're using three heat lamps. Heat lamps will suck the humidity right out of the enclosure and you will be struggling with bad sheds forever. For that type of enclosure, I would probably recommend a radiant heat panel hooked up to a thermostat.

You should also get rid of the analog dial type thermometer and hygrometer. Those things can be VERY inaccurate and your temps and humidity could be WAY off without you even realizing it. Not to mention that right now you're measuring the temps and humidity toward the top of the tank, not where your snake is actually spending his time.

Look up the care sheet on this site, read through it thoroughly and adjust your setup accordingly. I don't mean to sound harsh, but there are several things wrong with that setup that you should fix ASAP for the good of the snakes...