Both my cats third eyelids are showing!

My cat Bentley's third eyelid started showing about a week ago. Not a whole lot but definitely noticeable/more than a "normal" amount, so that you can always see the white of it even when he is alert, but not in a way that it is covering a lot of his eye or anything. I took him to the vet and he was checked out and he had bloodtests done, all normal, I am bringing in a stool sample of his later this week but the vet said he seemed fine and stuff. Besides the sleepy eye look he has he is being himself, no other symptoms.

Fast forward to this weekend, it's still showing. I let him run around and play with my new kitten Bitsy who he has been separated from for a month plus (she had a few URI virus' but never showed a third-eyelid and is all better now) for about 30 minutes. I separate them and take her back into her safe room and all the sudden her third eyelid is showing too !! The same amount as Bentley's. So now they both have their third-eyelid's showing (bottom corner of the eyes and top corner as well sometimes) and I highly doubt that if it was from some sort of condition/disease that it would have shown up on her that fast. I am just stumped and I have no idea what is causing it. Since the kitten never showed it before I am assuming it is being caused by something in the main part of the apartment that she has not been in before and where Bentley has been while they have been separated... but I have been living here for almost a year now with Bentley and have never had this issue before. It does get a bit dusty in here and I have also been sick so maybe it's from me but I am not sure if either of those things could bring that on so quickly?? And if it was due to my being sick the kitten would have shown it a long time ago because she sleeps with me. Any ideas as to anything I can do or what could be causing it?? I am going to look into getting an air purifier to see if that helps and stuff but I really am at a loss here.

Gosh, I have absolutely no idea, but it's definitely weird.  I would normally say they should probably be seen by your Vet, but since the older one already has... .  Maybe, though, now that the same exact thing is going on with the kitten since they've played together, it's worth a call to the Vet to explain the situation and ask if it could be an allergic reaction to something in the main room or something?  Or if the kitten starts acting odd at all I would definitely take her in.

I have no idea either

I have been manically trying to get all the hair and dust and stuff off of EVERYTHING here, still not done yet, but just trying to get rid of anything that could possibly be causing any irritation, ect. Just turned the heat way down to see if that has something to do with it.. left the deck door open a crack overnight to get more fresh air in, also threw out some of his old toys (he's gonna be mad at me for that one)... The kitten is due in for the vet in the next week or so anyways since she is almost done her medication and he wants to see if everything she was dealing with prior has cleared up, so I will make sure to mention it. Otherwise both of them are still acting like their normal selves...There has been no real change in the household that could have brought this on... so confusing =/