I don’t know the rules of Miraculous at all outside of what I’ve gathered from tumblr, so maybe this is illegal in canon but

I’m highly entertained by the idea that like, the class takes a trip to somewhere high-security that requires passing through a metal detector. So they tell the kids that all jewelry needs to be removed. Which is like, cue small amount of panic from Adrien/Marinette as they remove their ring and earrings. Some other patrons see their anxiousness and assume it must be because those earrings/ring are highly valuable.

And this includes the akumized person in line who’s here to rob the place.

Kids head inside. Akumized person overpowers the guard, grabs the jewelry basket, rushes inside to steal whatever else. In their hurry, Adrien’s ring drops out of the basket.

Marinette notices the person first. Hands shoot to her ears, but nothing’s there. So she races back to the security desk to retrieve her earrings. Lo and behold, they’re not there. There’s only a dropped ring on the floor and…Chat Noir’s miraculous? Plagg sees her, and all rules-of-hiding be damned pretty much tells her “Yeah I have no idea where Chat is. And that person in there is akumized. And you already seem to be aware of this so just put on this ring I’ll walk you through what’s about to happen.”

Very confused, she puts the ring on her finger. And (to Plagg’s confusion now) Marinette activates it entirely on her own, and becomes Mari-chat Noir (this is weird). But there’s not much time to think about it. Plagg just gives her the speed-course in Chat’s powers and she rushes back inside to fight.

The fight, of course, gets Adrien’s attention. Who now sees the rather clumsy battle between akumized person and “Hey that’s not me”!Chat Noir. Mari-chat Noir lands a kick on the person, which sends the jewelry basket flying. People start running from the scene. The basket lands next to Adrien. He drops to his knees and starts sifting through it frantically for his ring. It’s not there. What is there is Ladybug’s miraculous.

Tiki’s response is more or less. “You have to go help Ladybug up there. Are your ears pierced? Never mind. Just take these earrings and clip them to your shirt I’ll walk you through what to do.” So Adrien does, and (to Tiki’s surprise) he knows how to activate the earrings, and becomes Adri-bug.

Adrien joins the fight. The first 30 seconds are taken up by mutual “Hey that’s my miraculous!”/”Where did you get that?”/”If you’re Ladybug, and I’m Ladybug, then who’s flying the plane?” exchanges. But hell, at least they’re used to working together. Except now they have to coordinate by giving each other commands for the best Ladybug/Chat Noir technique, now as Adri-bug and Mari-chat Noir, to use in this situation.

The students watching the fight are downright baffled because it’s definitely Ladybug and Chat Noir up there. Except…Ladybug is Chat Noir. And Chat is Ladybug. And describing the fight becomes downright impossible because it boils down to, “Then Ladybug whacked the guy in the face. Well I mean it was Chat Noir. But Chat Noir was Ladybug. Okay I mean Chat Noir the person was Ladybug the superhero and he whacked the guy in the face. No wait, let me rephrase.”

After the fight’s over, both of them agree to shut their eyes, remove the jewelry, and drop the earrings/ring back into each other’s hands, and turn away. It wouldnt be right to out each other over this. Both Tiki and Plagg go home with their proper owner. And both of them are quiet in the realization of what happened. Because yes, that was both Ladybug and Chat Noir fighting just now, and both Tiki and Plagg know exactly which classmate they flagged down to take on the opposite miraculous role.