Well, to be fair, I really think you should include some female AB's to give a different perspective from just the guys. I can understand it being a bigger jump for a guy to break his masculine stereotype by regressing but taking into account some female perspectives would be awesome. Perhaps what you can do is show ways the characters learn acceptance (maybe by using a fake support forum similar to ADISC?) and connects with other ABDLs (since it's rare for two AB's to discover each other at a school or something) and as one of the characters gets in touch with other AB's online or at support groups or whatever, you can have glimpses into those other people's lives to offer different perspectives that help the main characters find their own acceptance. Maybe the two main characters never have to directly interact at all, and you can simply show a parallel between the two lives in totally different environments as they figure out their own lives and journeys.
What I would say is subtlety is going to be your friend with this video. I would illustrate both guys' lives in the normal course of daily life where AB/DL isn't even brought up or considered. And as life goes on and the audience is connecting with this seemingly normal person, small parts of the AB/DL lifestyle make subtle appearances. Like the elephant in the room that nobody wants to bring up. The conservative guy could have a lot of high expectations put on him to succeed in school, sports, life, etc. and maybe as he makes mistakes and beats himself up over it, you see him sulk into his room, lock all the doors and become very shut in, then reach for a teddy bear and pacifier to help him sleep. Or maybe he is so conflicted he doesn't even have a teddy bear or paci yet, and so bundles up a blanket to hug and sucks his thumb.
And maybe the 'liberal' one hasn't reached full acceptance yet but is much more invested in the lifestyle though still hides it. Maybe he is much more childlike and dresses up young for his age and you can see him sort of hinting at regression in his normal life without being overbearing. If you make the contrast too stark, then the characters will be off-putting to the audience. You need the vagueness so that people can fill in the blanks on their own and connect better. By just alluding to the fact that both characters are hiding something embarrassing and that the stresses of life are beating them down which forces them to turn towards whatever they're hiding, a lot of non-ABDL's can identify with those aspects of life. Once the audience is invested in these characters' lives and they are chomping at the bit to figure out what it is that is their dark secret, then it almost won't matter what the vice is. But you have to make that universal audience connection before you can start blasting out the ABDL stuff.
I honestly wouldn't like the Matthew character the way he's described as having this idealistic well-off life where now that he's accepted himself and all his friends are accepting, life is just grand and going great for him and he can be the superhero that saves Andrew from his own self-loathing. That's not a believable character. Maybe give him some really cool friends and a few of them might know about his ABDL side, but otherwise you run the risk of making him this ABDL poster boy that is far removed from the harsh realities of life. If anything he should have a wide variety of background struggles going on that he can now overcome because he's accepted himself, but accepting ourselves doesn't remove the issues entirely. Maybe it can seem like his world is all going downhill on a wide variety of levels but he finds a peace and grounding in the fact that he has accepted himself whereas the other boy can seem like he has everything going for him but is having a constant internal struggle to find his own identity when society seems to be giving him an identity that he doesn't fully embrace.
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To further drive home the subtlety point, maybe you shouldn't include an infantilism reference in the film title at all. The name should really convey something that makes people want to see what is going on. If you can keep people interested, invested and guessing, then you've got a great start to your plot. Once you've got the audience locked in, don't be too easy about the whole thing where you just dump out the main characters personal lives like a book bag and say, "Oh, you caught me! Guess I better tell you everything about my ABDL side and what I like and dislike about it." That's too simple and insulting to the audience's intelligence. The audience will lose interest once it's been laid out like a Bond villains evil plan. Make the audience connect the dots and figure it out for themselves. And just keep making small references to the bigger thing. The whole point should be to spur discussion among the audience. Otherwise, if you just want to be factual, you're better off making a documentary. A drama has got to be convincing that this is a real life going on and it has to connect the audience to the character's whole life and personality. It should be a film where any non-abdl can look at it and go "Oh man I can totally see myself being in that situation. I get where that character's coming from now." Create the empathetic link first and foremost.