Q.Dealing With Christmas Cactus Root Rot
I’ve had two Christmas cacti in an 18″ pot for over 10 years (smaller pot previous to that). One of the cacti got root rot and I have pulled out the plant from the planter. The other one seems not to have been affected. I have a smaller Christmas cactus that I’ve been growing for 2 years and is in a 6 inch pot. Can I plant the smaller plant in with the older one in the large planter? (Actually, I already did; but then had second thoughts if this was a poor idea.) Your recommendations would be greatly appreciated. Photo attached. P.S. It does seem that photo uploaded.

Certified GKH Gardening Expert
My concern was using the same infected soil. If you didn't change the soil, I would do that and clean the pot well too (1 part bleach to 10 parts water) before refilling. It needs to be loose and well draining such as a cactus mix.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/repotting-christmas-cactus.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/christmas-cactus-root-rot.htm