I'm not really understanding the problem here. How would the Apostle Twins sharing their twins ability defeat the purpose? They are born from the same source, the Arbiter. You know, the being that has the original trait of the twins combined? It's not even like they're experts with each other's ability. The 4th is great with her Memory Trait, for obvious reasons, and yet the 3rd sucked at it. Them being able to pass on their twin trait isn't even a big deal, going off of Ella's and Aglaecwif reaction. Also, what does the Apostle Twins appearance have to do with anything? There's nothing that says the twins have to look a like. Even in real life twins don't have to be identical. That seems like an odd point to try and make.
As for you dying on the hill, why would I care? You're allowed to do whatever you want, just like myself. Going with what's shown in game though, we see only one twin infect the MC. We see that one acknowledged by everyone that comments on it and only see that one during majority of the scenes that involve the parent showing up. The MC only refers to one as Mother or Father. Even in the scene with both of them. Aglaecwif also cosigns Ella about Jake being able to get Order, which requires the Power Trait from the 2nd, and we know he's only the spawn of the 1st.
There's also a huge flaw here and I'm surprised you didn't realize it. Why didn't Ella get the Memory Trait if "team work" is the only requirement? Was she unlucky? But according to you, there shouldn't be any luck involved. Ella expected the Memory Trait at multiple points and yet she failed. So is she just the spawn of the 3rd? But if that's the case why would she expect the Memory Trait if team work is what made the dream work? Or is she also the spawn of the 3rd and 4th? But then why didn't she get both if all it takes is "team work" from them? Why wouldn't they work together for Ella, who was also compatible but do it for the MC? Sound like luck does play a part and that the MC is special, regardless if you believed that both of them were required.
Hell, why not have more spawns with both traits if all it took was them working together and infecting the same person? So, pick your poison, I guess. Either Ella is only the spawn of the 3rd but she felt like she could get the 4th's trait, which contradicts you since she would know better than us and you said it doesn't work that way. Or she's the spawn of both like the MC but didn't get it regardless of them working together, which would still go against the notion that both infecting a single person gives both traits. Either way you go, it would contradict your own view on it since there's conditions for it all. You can always take option 3 and say Ella was just wrong but then you'll have to say that about a lot of things she said. Especially since she claimed to know more about monsters than any other human or superhuman in game.
Lastly, there's nothing in the story that, "narratively" contradicts getting both from one twin. Monsters pass on their "genes, traits and powers" via infection. It's already confirmed in game that the 3rd and 4th share their traits. If power and traits are passed on via infection then why wouldn't the 4th, who has the Memory Trait and a certain level of the Body Trait, pass on both, if they're compatible enough? Same with the 3rd, who we know has the Memory Trait but is bad at it. The MC having a perfect balance means nothing when his traits were at the very bottom of their growth. It's like saying a Novice Mage with fire and lightning spells is perfect compared to a Expert who has fire mastery with lower lightning spells. Of course he'd seem perfect when he's wielding low level spells currently.
Tldr: Basically, this is being needlessly over complicated when the game shows you plenty of reasons for it to be multiple ways. There really shouldn't be a hang up here besides ones own personal belief getting in the way.
It's not really a theory, to be frank. Ella already confirmed cross inheritance is a thing when she saw that the MC only got shapeshifting and said "I thought as much" even though you're a spawn of the 4th. Her and Aglaecwif thoughts on Jake and inheriting both traits lead to confirmation as well.
You don't really have to look at the 7th. Though I did point it out that he used Light to kill the MC as supporting evidence that the 8th is Light. All you would have to do is look at the 3rd in the beginning after trying the 4th. It's clear he tried to read the MC's Memory but couldn't get it to work without hurting the MC so he used his Devouring Tentacle instead. MC later retells it when talking to Alice and Clark about how he was turned.
The 7th having Light, in some portion, just proves that the Twins, even outside of the 3rd and 4th, share their traits. Even when they don't work together. Could it increase in potency if they did, or the likely hood of their spawn getting both traits? Possibly.
Interesting take. One I haven't really thought about, admittedly. That still would mean only one, potentially, infected the MC though. Even in that scene when they used their injection, you'll notice the color matches the color of themselves and not a mix of both. The 4th's liquid in her injection is pink, while the 3rd is bluish purple. If the purpose was to give both via that method, only one was required, since they did both and the twin didn't do the other.
If both injections had the same color or the opposite twin came and did it than I could see why one would think they both mixed theirs together. But in game, only one did both and the color matched the twin who did it which would prove only one had a hand in it, not both. If this method is accurate.
Anyway, that ends my ridiculously long post. You guys are going to have to suffer this wall of text because I didn't spoiler shit.
