Fun Riddle

Er, are we assuming he is cutting part of the rope off?

Because if he climbs halfway up the rope and then simple is able to pull the whole thing down he could steal all of it.

If it's to whatever he can reach, he can steal half of the rope + the length of his arm

Almost all of both of them, depending on how they're attached to the ceiling. Tie the ends together on the ground, climb up to the ceiling. Hold on to one, cut the other one a foot or so from the ceiling but don't let it drop. You're now hanging onto the top of a double-length rope, next to a small rope. Tie the small rope into a loop, and feed the long rope through it until it's halfway through. Now grab onto both halves of the long rope on either side of the loop, and cut where it was originally attached to the ceiling. Slide down the (now folded in half) long rope, and pull the whole mess down after you. If you assume there's some sort of beam or hook up on the ceiling already, you can do without making your own and get both ropes in their entirety. No falls are necessary at all.

since they are close together he can steal all of the longer one and half of the shorter

both ropes.
If he climbs to the top of the ceiling, he can then grab on wherever the ropes are being hang, and then untie them from the ceiling and tie them to each other, now you have a big rope.

Now you just make a big U (inverted) and climb down from the ceiling using both sides of the U, and when he's at the ground he can just pull one of the sides and it will scroll over the ceiling and he'll have the full length of both ropes.

The problem is the that someone who steals pieces of rope as their full time job isn't smart enough to come up with something like that. I think they are more likely to climb 3/4 the way up one rope, try to scratch their arm and their leg at the same time, then fall to their death with no rope.

How is he holding onto the rope while tieing the knot at the top of the rope?

Well I imagine if he squeezes his legs together hard enough.. :p

Couldn't he be climbing all the way up on one of the ropes, cut the other rope off at the top. Then continue by climbing half way down on his current rope and slicing that off, falling to the ground. Leaving him with 3/4 of the amount of rope. :o