Forget Zach and Zorian trying to lay low about their magical abilities. What about Zach and Zorian trying to keep everyone from confusing the dynamic of their relationship??
Silverlake literally says that the only explanation is being gay and I know she spent quite a lot of time with them but people (especially teenagers) always seem to come to the conclusion that "you're close so you must be dating" so even though Zach dropped out and let's be real, Zorian will probably end up skipping a lot, people will still see them around and be like, huh, they are weirdly close... The only real edge ZZ has here is the fact that they are two boys, not a guy and a girl.
There was the whole going to Daimen thing together as well where Daimen was being a little shit and went "aww this your bf? <3". And Kirelle's opinion really mattered to Zach (after she initially didn't like him in one of the loops because of the "train station incident"), plus he managed to loop her into his petty revenge. Fuck the rest of Zorian's bio family. Alanic's opinion probably also matters a lot to Zach too. Seeing as he is Zorian's father figure and all. Plus their friend.
They have chemistry. It's not necessarily a romantic one but it's there and it's really obvious + the soul bonded thing. They've been friends for so long in such unique circumstances it's hard not to be super close.
Adding to above though, to everyone else it looks like they became besties out of nowhere which is. Not possible. At least with romantic relationships you can say "omg, first relationship, so smitten". Friendships take time to build. From the outside it would definitely look like they have crazy chemistry for a romantic relationship and that this isn't just the results of a ~decade long friendship.
They are a team. It's ZachAndZorian. ZorianAndZach. It's all "us", "we". They are constantly together.
They have the whole opposites attract thing going on (sun/moon, golden retriever/black cat, red oni/blue oni etc.)
They hang out all the time. They eat out together. They go to the bar together. Everything. They literally do every single last thing together. And somebody like Benisek would definitely notice and be like hm, you've been going to bars together... those are called dates. You are dating.
They leave stuff at each other's places probably. Like "his house is my house, anyway" pretty much. Also Zorian lives with a bunch of other people and tutors people so he can't leave incriminating stuff around at his landlord's.
They share their money. And it is their money. It isn't Zorian borrowing Zach's money or vice versa, it's quite literally theirs as in ours. As in "my money is your money" which. Isn't normal for best friends. Like listen, I lend money and borrow money from my mates sometimes but it isn't an "our". For them, they share funds.
Plus they are definitely touchy. Zach just strikes me as a "your personal space? Our personal space" guy, plus the whole Zorian faking his death thing, and Zorian doesn't really mind either way.
Their futures include each other, they're not planning on separating. I feel like they'll definitely move in together at some point.
They are each others go to person. "I know a guy" literally is just them talking about the other. Which is fair because if Zach can't do something then Zorian definitely can (and vice versa), so it works. They literally really compliment each other in abilities.
They are also both very mature and close so they can talk about whatever without it ever getting awkward.