I once did this between 2 diff. plants I had. I used a very small artist's paintbrush to take pollen from one plant & insert it into the flowers of the other. It resulted in ONE fruit, sometimes called an 'apple'. Looked like a smallish round berry, which over time reddened into ripeness. Mind you, that reddened berry held on to the plant for abt 1 year, at which point I removed it. I broke it open & spread the jelly-like contents, full of seeds onto a paper towel & let it dry out a bit to separate the seeds from the mucilaginous gook around them. I did try to pot them up, but I think the seeds dried out too fast & nothing came of it. The tricky part to doing this is to have several plants in bloom at the same time. Now I only have one mature plant, so I can't try it again. If you do try it, you might try doing it several times a day, at diff. times, say both morning & night to improve the odds of success. Good luck w/ it!

Turning red suggests heat and/or water stress. The plant may have lost its roots through overwatering; tip it out of the pot to see if it has any. If not then you will need to reroot it. Check your potting mix to ensure that it drains freely and does not stay wet for a long time. The mix should contain components such as charcoal or vermiculite that act as reservoirs of moisture without excluding air from the root zone. (Some growers use kitty litter, the heat treated clay variety, for this purpose).

Let them flower and then do the dirty!! I have repotted and chopped, but the best results I saw came from old plants. Dead easy. Just take a piece at least 2 sections long, you can dry out a bit first, and then repot into pots or baskets. I have a broken bit from an old plant which is flowering away on the potting table. When it finishes I'll take cuttings. As long as you keep new ones it won't matter when the old ones die off. Mine wither and rot at the base.

I am still in search of True Christmas Cactus as well as Plumeria and have the following plant cuttings for trade - Zygo Cactus (Orange, Pink, and Fucia), String of Bananas, Swedish Ivy Begonia, Graptoverias (Fred Ives), Graptosedums, Jade, and Hedera Helix Ivy (Ingrid Liz). Just let me know if you are interested.