After this update, I don't blame them for not working together. They were basically like, "So you're saying we're only one half away from being complete and ruling over everyone? Well, it looks like it's time for you to get in my mouth, dear brother" or something resembling that of sorts.
But seriously, the reason why the 3rd and 4th probably work together is because those siblings care about each other. It could also be because their Papa Arbiter is still awake and they had no reason to fight each other as a result. Hell, that's probably also why they have a decent amount of control over each other's traits as well. Since the Origin is still there, they have no problem with sharing and working together. As for why the 3rd is implied to go out there infecting everything compared to his sister, I have no clue. Maybe the Body trait isn't as strict while still having a decently high ceiling? Or maybe Mommy 4th wanted a real pervert as her son, on top of being qualified to be her spawn.
Meanwhile, the 7th's crazy ass is a menace to Apostle society. I was right about that lunatic hunting the Numbered specifically. Hell, even my theory about him being the lesser form of a greater god was partially correct and it's highly possible that unlike the other Twin Apostles, he actually is his own Arbiter. Instead of his Arbiter being sleep or missing, it fell into the 7th and they fused. Probably already tried to eat his twin sister or brother, The Light. Then again, he did have a bride so that probably triggered all of that hatred he has in his dark heart. The Apostles probably slaughtered her and he wants to get it back in blood.
Not going to lie, I might join the homie Val and paint the universe black for our Lord and savior, Dark. My Murderhobo is a lost cause anyway.