Wow, that's pretty old for a Plumosa, Yukifoo! I would prune off all the dead stems, and cut the live ones back by 2/3--i.e., if the stem has 9 "plumes" on it, cut it to the point where it has about three. take the plant out of the pot, and carefully examine the crown, where all of the stems sprout from. Usually, you will see several tight groups of stems, with some space between them. Those are the natural places to separate into sections. If you can pull them apart with two garden hand forks, that will result in less shock to the plants. Often, the roots will be so packed and tangled, that you will need to "cut the Gordian knot", and be done with it. I would then repot the sections, no deeper than they were originally, water them with a good root stimulator, such as Superthrive, and and keep an eye on them. New sprouts in a few weeks means success!
Good luck!