Whether or not the plants are 'choking' each other would be a function of how congested the roots are in the entire soil mass. Root congestion restricts growth because the soil becomes packed with 'conductive' roots, roots the primary function of which is to move water and nutrients, rather than to gather them. This leaves a reduced volume of soil to support the fine hair roots that do all the real work. The 'center' you refer to would be called the 'crown'. If the roots have filled the pot and are restricting growth, you should consider repotting/root pruning or separating and repotting the plants in the future, but winter is a poor time to consider work as major as that. Instead, wait until early summer when the plant(s) have gained some energy and they are better able to tolerate the stress. If the smaller plant is an offset from the larger, you can simply cut them apart with a longitudinal cut so each has a healthy portion of the root system. Dust the cut with powdered sulfur if you want, to help thwart any rot organisms that might want to wreck your day. If the plants are individuals, tease or cut the root mass apart to separate the plants & repot. Don't be afraid to cut the plants back and prune roots. This type of treatment forces growth from cells closer to the root systems, which are ontogenetically younger (juvenile) and more vigorous, which is where the term rejuvenation pruning comes from. Al

I haven't seen that problem before but was wondering whether it might be from something in tap water. Rainwater is best. They're an epiphyte (and lithophyte) from rainforests, mostly growing on trees with no soil and roots exposed to the air.

I had almost the exact same thing happen. I had a bird's nest fern for a long, long time. It was out-growing the pot, It did well in winters underneath the oak so I thought, "Why not just plant it under the oak?" I did so and mine started to follow the same path as some of those discussed above. After watching it get worse and worse, I finally lifted it and replanted it into a new, larger pot. Almost immediately it started to get better. So ever since, it lives in a glazed pot under the oak. It sure didn't like being in the ground - or at least, my ground. Carol in Jacksonville

I have no idea what native potting mix is 🙈 Sorry, I do want to help, but that's hard if you don't understand bits of the question 😅