Climb to the top of one of the ropes, cut the other rope at the top, cut the rope you're hanging from at the top as well, and fall towards an uncertain fate at the floor.
If falling towards uncertain fates aren't allowed, keep a small portion of the rope for you to cling to when you cut the rope you're at. Then cut a hole in the roof with the knife and climb through. If the roof is very hard and the knife becomes dull after a couple of days, use your fingernails. If cutting the roof is too cumbersome, go up to the next floor and cut a hole in the floor around where the ropes are in stead (the latter, however, is considered the easy way out, and is frowned upon).
If that doesn't work, bring the roof down by pulling at the ropes, escape before it hits you, and return later to collect the ropes.
Or, of course, you could just throw the knife at the top of the ropes to cut them, but that'd probably be too easy.
Alternately, cut off a portion of one of the ropes, and tie it back so you've got an Y-shape. Hold this, and climb to the top of one of the ropes. Tie the ropes together in a knot in such a way that they cross each other, ie. the rope on the left ends up coming down on the right. Now place the knife above this knot with the sharp side down, tie one end of the Y to the hilt of the knife, and carefully tie the other end to the blade (wrap it around a few times to prevent it cutting too easily). Climb down using the rope not connected to the knife. Now, either catch the other rope when you hit its bend (should be about 10 feet from the floor) and pull, or go down to the floor and jump up to catch it.
Huh, that last one could possibly even work. (Okay, I'll stop now.)
Edit: Eh, forgot that the ropes are 20 feet apart. I guess you could swing... Not that I meant any of the above seriously, of course
[Edited by trick on 01-30-2004 at 01:17 PM GMT]