hi there ! the grey cockatiel should be a male, but i'm not sure about the yellow one. you need to observe the behaviour and take a look at its tail for any patterns as well. you should also bring the yellow cockatiel to an avian vet since it's plucking and you can see its skin.
are they tame? and what kind of diet are they on? if they're only on seeds, i'd recommend slowly introducing some pellets into their seed diet. you can also try having them eat veggies by having chopped vegetables in a separate food bowl and sprinkle seeds on top. do this every day, start with a decent amount of seeds, and add a smaller amount after 1 week. i would repeat this for maybe 3-4 weeks? add lesser seeds every week! they'll get used to it and they would somehow eat a little of the veggies while trying to eat the seeds and they will eventually begin to accept veggies as nice food :] that's what i did with my cockatiel, and he willingly eats vegetables now :]
i would also recommend adding some bird toys into their cage. the cage is not very visible in the pictures you gave, but i hope their cage is large enough for both of them too. one last thing, i think you should try letting them bathe. placing a shallow water dish in their cage maybe? or you could try using a spray bottle, and spray towards the sky and let the mist drop onto them.
please feel free to ask more questions! the people on this forum are all kind and willing to help whenever you need <3 all the best with your feathered friends!
if anyone has corrections to make or more to add on, please do reply!