Answer: Although your yucca is occupying a single pot, you have two plants; one is thriving, the other is suffering from some type of root rot. I know it sounds impossible for one plant to exhibit healthy growth and the other to be in a stage of decline when they are getting the same water, light, etc., but the truth is, one of them is struggling. If it were my plant, I would unpot it and carefully cut the ailing plant away from the healthy plant. Repot both in fresh potting soil. The healthy one should be no worse for wear after being repotted and the ailing one may recover once it is in its own pot and doesn't have to compete for moisture and nutrients. It's worth a try1
Best wishes with your yucca plants!
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