Q.provide help in splitting foxtail fern
How to split a foxtail fern plant? This fern is from California, from a relatives garden, a small cutting which fit into a 6-inch pot. It was divided 6 or 7 yrs ago by the nursery in our town. Two plants exacting the same size. One is still alive and now it needs to be divided, and its twin died – same light on them and same care.
That is curious isn't it - one twin dying, the other flourishing? Probably some disease some fungal spores got into the one pot, or were already on the plant when it was divided. Now you're ready to divide again. First question, are you sure you want to divide (in other words, you want to have two plants instead of one,) or do you want to up-pot (put the plant into a larger pot so it can continue to grow larger?)
If dividing is what you want, it's not difficult. Take your plant (not a fern, of course, more information here: https://www.gardeningknowhow.com/ornamental/foliage/asparagus-fern/foxtail-asparagus-ferns.htm ) out of the pot, and simply divide it with you hands and a sharp knife. Here's an article to help you: https://www.gardeningknowhow.com/garden-how-to/propagation/propgen/dividing-plants.htm
After it's divided, repot the two pieces, spreading out the roots, you can probably re-use the original pot, and another the same size. Here are some pointers on repotting: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm