Hey, I've really enjoyed this game, can't wait to buy the final version ! Is that still planned for October 30th btw? Also have you thought about maybe shipping a Linux native version? If so and you need someone to beta test I can help with that.
That aside, I've got some feedback:
First off, you've found a truly awesome formula with the splitscreen stuff, I'd played some of your prior games and they showed potential but this new approach really feels like you've struck gold. And for a first go at this setup you've done it as well or better than other games with similar concepts. I hope you'll be sticking with that general idea, tweaking it as is appropriate for the different settings you might want to explore.
That said, it's not all perfect. It works best when you're basically chasing after Aria to catch her doing naughty things, but that aside the overworld is kind of aimless. Caellum doesn't really have anything to do that requires him to walk around, we as the player only do it because we want Aria events to spawn. Which also means that if we don't want to chase after Aria to keep a fully oblivious Caellum, there's practically nothing to do but standing around while stuff happens to her.
Someone mentioned a "streamer girlfriend" idea which does sound extremely fun but considering that likely wouldn't have combat/dungeon parts, going with that setting or something similar would really require implementing overworld quests for the MC to do. It doesn't have to be super deep, just give us reasons to go from point A to point B (going to buy groceries, the girlfriend has asked you to pick up some takeaway for her, going to University or working odd jobs, ... whatever fits the setting) so we have things to do while the FMC is being lewded. And then if we want to take detours from those quests to see what she's up to in person then that's fine too.
Another more minor thing, IMO it would be nice if we had a mini map to more easily locate where the FMC is.
Anyway, good luck with the final stage of development !