Here is the photo of the second dying cactus:

I have primarily succulents but I think their care is similar. I use Miracle-Gro Moisture Control soil for my plants and they respond very well. I would recommend repotting into some sort of similar soil, you can even mix it with some rocks or vermiculite and make sure it can drain. The soil will provide nutrients, just make sure to let it dry out completely before watering. Do you keep these where they can get plenty of sunlight?

Ok thanks for the advice!! So far I have just left it in the soil it came with, which is kind of a grainy soil on the bottom and a layer of sandy gravel stuff on the top (cactus sand). I've wondered about fertilizing it but I heard you shouldn't do that in the winter.

I also have an aloe vera plant, would you recommend repotting that one in this soil as well? The aloe vera seems to be doing ok and is in some kind of clay-like soil. Also another random question, how often should I water the aloe vera?

My question is though, if these cacti already have root rot, is there a way for it to recover or is it already too late? The dying cactus, idk if you can see from the photo, but it has a brownish "tough" tissue thing on the bottom, which looks like it COULD be corking, except that type of tissue is also on the top of the plant and in spots along the middle, and the middle green part is turning light green from the top down.

Also from the photo do you think the barrel cactus has root rot or is that just corking/normal discoloration? It's kind of a brownish tinge over the normal green color of the plant, going about halfway up the plant from the bottom up. I did take it out of the pot about a month ago to check for root rot and didn't see any wet or rotting roots...it did seem a bit rootbound though. I went about a month without watering it after that because I didn't want root rot to happen, could that have caused the discoloration?

I keep them in the living room which has three west-facing windows and I always open the windows in the morning. In the summer they got a TON of sunlight and the dying one even got sunburned on one of its stalks. The barrel cactus didn't grow at all (hasn't grown since I got it) and the dying cactus only grew a tiny bit, but my third cactus, which is one of those cactuses that looks like a bunch of ovals stuck together, seemed to love the sunlight and grew a ton. Now that it's winter they don't get quite so much light but it's the most natural light they can get in my apartment.

I am a first time cactus owner so I really don't have much experience with any of this stuff, so any advice will be greatly appreciated! Thanks!!

If you're going to use cactus soil, you should mix it with atleast 50% PERLITE. Vermiculite HOLDS water. Cacti and succulents need excellent drainage. They can't stay moist for too long. Are the brown spots mushy? Or soft to the touch compared to the rest of the cactus?
If you're feeling ambitious, you could Al's Gritty mix in the search bar on this forum, it gives you his recipe. It provides EXCELLENT drainage. All my plants that I've switched are doing great. Though some people may think it's too much work. You should read up on it and see if it's something that would interest you. If it's not something that you want to try. Half cactus/succulent mix with 1/2 perlite should work I would probably do 30% cactus soil and 70% perilite myself. Hope this helps! :)

No as far as I can tell they aren't mushy or soft...although of course I haven't spent much time touching them since it is a cactus. When a cactus gets brown at the top that usually means it's too late though, right?

Can I find the supplies for this at a Home Depot/other type of garden center?

Will definitely look up the gritty mix and if it's too expensive/time consuming I will try the other mixture you recommended. I know literally nothing about soils so it's great to get the advice of people who do!

You can get cactus/succulent soil and perilite at any big box store. Sometimes cacti and succulents get what's called edema from over watering also. If it were mine, I would pull it out and check the roots looking for signs of rot, and then repot in dry medium of the mixture I suggested. I wouldn't water for at least a week. Then only water when the soil is dry. You can use your finger to test the soil, a skewer, or a pencil. Is it possible to get a close up of the discolored parts please?

Ok, sounds good. I watered them for once a week for a couple of months, then didn't water for a month. Then I watered them two days ago but used a bit less water.

I pulled them out looking for signs of rot about a month ago. The dying one had rotting and wet roots so I removed all those, along with the rotting stalk, and put the healthy roots back into the soil. The barrel cactus had no rot but looked a bit root-bound.

My camera is awful but I'll see what I can do about getting better pictures.