Super-duper grower reference book to the rescue!
Sort of! Almost! Possibly! The timing is all wrong, but otherwise I think you have what the abovelinked book calls "head breakdown." Check it out: "Another common disorder is head breakdown, where yucca canes prout heads that may grow a few inches, then begin to die back. Heads become pale green and withered looking, with tip burn and numerous water-soaked lesions in the leaves. This is also caused by a simple failure of the cane piece to root. The head comes out, and the roots intended to support them fail to develop. Discard cane pieces that show these symptoms." Now, that's not a stunningly close description of the situation, but it's close enough that I think we can maybe work with it. (And besides, all the other diseases and problems he describes are further off.) Have you looked at the roots? If not, you may want to take the plant out of its pot and see what's going on down there. (Take pictures while you're in there, if you can, 'cause I'm betting that's where the trouble lies.) If there's a smell, mushiness, blackness, etc., then we have a culprit. We're also looking for how much root development there is -- if I'm reading the above correctly, this could be because the roots failed to grow in the first place, or because they grew and then had problems of some kind. If the roots seem to be fine, then I'm kind of stumped. The book also mentions that canes that sprout lots of heads (which it looks like the larger one of yours has seven: this is a lot) "will fail to develop adequate size." Which is vague enough that it's open to interpretation, I think. One possible way to go is, if the roots are abundant and healthy, and the cane is still firm and solid, we could try cutting the tops off of both canes and see if the plant can be induced to start over, on the theory that "fail to develop adequate size" means "fall over dead." This saves most of the height of the plant and might end happily. If the roots are abundant and healthy, and the cane is mushy or hollowed-out, then you have rot of some kind and will need to take more drastic measures. The most likely way for that to play out is that you'd separate any cane that's still healthy, cut it into 3-inch segments, and try to root them. You might not want to do this (it's a little like saving the village but losing the war), but in terms of salvaging something, it might be all that's left to try. Standing by for the root report, mr_subjunctive | 
