This is when Steven and Connie fused into Stevonnie and fight Infinite in Disharmony: Gods Among Us.
(we see Infinite growling)
Infinite: More ants to crush. I wonder if Equestria can offer me a real challenge.
(Then, Steven and Connie, arrives but then sees Infinite, as he turns and sees them too)
Infinite: And if it is to answer, the world sends me a bunch of trembling children. (Goes to Steven and Connie, as they are scared) Are you gonna fight and die here, or run away in fear again?
Martian Manhunter: (in Steven and Connie's minds) He is unaware that the two of you can fuse. Wait for the right moment to do so, then strike.
Connie: Steven, you thinking what I'm thinking?
Steven Universe: Yeah.
Connie: Don't be scared, Steven. You're one of the crystal gems. And they believe in you.
(Connie hugs Steven, and then Steven and Connie fused into Stevonnie)
Stevonnie: Let's fight!
Infinite: A fusion? How did they-? Fine. I'll teach you fear and pain, and then. Well, at least the fear and pain will end.
(They begin fighting)
(Stevonnie swing her blades at Infinite, but he uses the Phantom Ruby)
(He knocks Stevonnie down, pinning her down)
Infinite: You the two individuals must be the allies of the Rainbow Forces. The strong will always vanquish the weak. You have your own frailty to blame.
(Stevonnie gets out her shield, blocking)
(Infinite uses the Phantom Ruby, but Stevonnie dodges)
(Stevonnie gets out her sword and shield)
(Then Infinite charges toward her as he misses)
Infinite: Well, well. It seems behind that fear is a spark of defiance. And those eyes, I feel like we've met before.
(Then, Stevonnie kicks him in the crotch)
Stevonnie: You don't know me!
Infinite: (falls down in pain) Why would you even do that?! But you put quite of a fight. I underestimated just how desperately the feeble cling into life.
(Infinite blasts her)
(the blast sends them into a wall)
(Stevonnie defused back to Steven and Connie, and Infinite sees Steven)
Infinite: I remember you. You're that kid that ran from me before. You survived then. But you throw it all away now. Curious. (Uses the Phantom Ruby to make lasers to aim at Steven and Connie) This will be good bye.
Connie: Steven, watch out!!
(suddenly, she saves Steven right before the lasers hit them and then the laser aim at them again, they hug each as they start to glow, making the laser pass them)
Infinite: Impossible. It can't be.. How did you-?
(he keeps attacking them but Nighlock gets in the way)
Nighlock: (off screen) Get away from them.
Infinite: No matter, that machine was just a decoy. There are only four days left. Despite of the resistantable, until Twilight and Celestia end this once and for all.
Nighlock: Well, that's why me and Batman rebuilt the Fabrication Machine and had it locate the real one. Right now it's in ashes.
(He flies off)
Nighlock: Let's get back to the hideout quick before the tyrants find us.