It does look like a case of overwatering. Hylocereus is a tropical cacti but it still needs proper interval in watering. With my cacti and succulents, I always water from the top and let the water drain out. Succulents hate very wet and soggy roots. OOps, editing to add..you were asking about what is proper interval in watering..it will vary depending on your growing area there. You just have to see if your media is still looking moist, then don't water. Allow to dry a bit but not too dry since Hylocereus is not a desert cacti. You can use toothpick or bamboo skewer, if you stick it in and comes out still wet, then no watering needed. Or put a big stone on the container beside your cacti, if you lift it and it shows moist below, then media is still wet.
Root mass of cacti is typically shallow, I would not recommend making your container too deep, since it will make more media stay wet longer. Your Hylocereus cacti is just growing as its growth habit wants it, getting longer, so it is not that the yellow cacti is getting too top heavy, it is the Hylocereus doing its natural growth habit, elongating. Usually it is just allowed to ramble about or some would even tie it to a post and it has roots that can attach to the posts or stems later on.
I do not know if you are willing to attempt to do a re-graft of the yellow one, choose a different cacti rootstock, something more upright growing and can carry the yellow better, both with similar watering needs too. But you may have to wait till it is warmer weather to do it.
Here are two videos about it and see if you are willing to try, good luck!
https://youtu.be/rikUpPDvu4g
https://youtu.be/eLCrMi88llo
