I dont think that "equip two of the same pet and they draw on top of each other" is acceptable to just implement as-is. Sure not all pets have the exact same AI, not all of them will have problems, but i think that two pets just sitting too close could already look off. not saavy enough on Pet AI to comment on the "eventually desync anyway" but i dont think having to wait on a desync is acceptable either
Better fix these then:
(there are plenty of other ones involving these pets but I couldn't be bothered to make a comprehensive list so just have the first few I could find, in particular a lot of the ones that try to move behind the character overlap with Crimson Heart and Fairy Bell).
A trillion accessories also do this already. Every hairpin stacked with each other in the past, and only recently was it made highly inconsistent by allowing you to stack some hairpins but not others with each other. If I want to look silly,
what is the harm in that. It is NOT out of character for this game to have weird vanity interactions like that, and pets are vanity as much as these are.
You can wear 3 overlapping capes at once and extend prince cape's shoulder pauldron with another pauldron behind it in this game for crying out loud:
I'm not convinced community backlash is as much of a concern as you're making it out to be, considering A) this is probably the first time you're seeing any of the above interactions B) every time I've seen something similar brought up, anyone who mentioned it considered it a
draw of the vanity system, and C) such interactions are far, far more numerous than what I listed here and that one """"edge case"""" I mentioned earlier. With that in mind, I can restate that this is less an actual issue and more a matter of taste. When something is purely aesthetic like this it becomes the only inarguably correct time to abide by the rule "don't like it don't use it" because there is absolutely no other reason you
would be using it.
You already have to wait on a desync for many pet + light pet combinations. If this is really as big a problem as you make it out to be then more work needs to be done to address that in the base game, and at that point just embrace it by implementing this suggestion, it would genuinely be less work than going through every combination of pet + light pet to figure out every single case it happens with and fixing those.
And if you're extra extra scared of perfect overlap then you could always make it so the same pet can't be in both slots but still allow all pets in both. It's precedented behavior for equipment menus to do, considering accessory slots exist, and most people probably would want to mix up their pets anyway.
More importantly, you can make the visual jank look good if you are creative enough but it's not a good idea to just, change something, knowing it won't work like anyone would initially expect it to, and leave it to people to figure something out with it. It will just be seen as a broken feature, you can't compare it to 1 isolated case like the obsidian rose layering
An unexpected or "broken" interaction does not necessarily need to be considered a negative for the game just because it didn't fall perfectly inline with its original intent. We have a lot of proof of that in vanilla with step-stool clipping, hoiking, smoky logic gates actually having a function despite the smoke effect clearly being intended to be an indicator for failure to output, and many others as simple as buying items from merchants to shimmer them for commonly used materials.
We even got dyeable weapons in the form of Critter Shampoo... despite the devs citing the projectile dusts system as a major reason no weapons would ever be dyeable for years, and that exact concern being extremely evident on several minions (most noticeable on both stardust minions and Imp Staff) after implementation. Despite all that, no one actually cared that dyeing your stardust cell red still makes blue dusts, and I have seen exactly 1 person complain about anything even tangential to that when discussing dyeable minions over the 5 years it's been in the game, which is further evidence that this concern is being blown out of proportion and that people just won't use a purely visual customization option if it bothers them in any way.
Additionally you're forgetting that this suggestion does not remove any of the old pet functionality. You can still equip a pet in slot 1 and a glowing pet in slot 2. In fact most people will probably still instinctively do that or even encourage it when discussing the game, as there's an argument to be made that having a more useful pet for light and a less useful one for vanity is the most flexible. This objectively opens up more cosmetic options and I am highly doubtful the addition of some minor purely visual jank that already is present in the game is enough of a downside to offset the many upsides here.
TL;DR most players wouldn't care about pet overlap. Even the people in this thread have either stated they'd be fine with them overlapping or given no opinion on it. The most preventative measures people would probably require to be happy would be controlling which pet is drawn on top of which, and that's as simple as making 1 slot take priority (which in turn is as simple as making the order they spawn not random when you log into a world, so no additional code needed as I'm pretty sure they didn't shadow-drop a patch that makes the pet and light pet spawn in a random order over the years)