Some fanfictions use a device or an ancient artifact as the means of merging the universes. It could be a powerful crystal that has the ability to connect different dimensions. Once this item is activated, it starts to bring elements of the Voltron and Young Justice universes together, causing characters to meet and stories to intertwine.
Well, in many fanfics, they merge the universes by having some sort of magical or scientific accident. For example, in one fanfic, a portal opened up between Camp Half - Blood and the Young Justice headquarters. The female Percy Jackson then gets sucked into the Young Justice world and has to adapt.
I really like 'Cross - Universe Bonds'. In this fanfic, the technology from Voltron is introduced to the Young Justice world, creating interesting plot twists. The author does a good job of balancing the different tones of the two shows. It's not too dark like some parts of Voltron can be, nor is it too light - hearted like some Young Justice episodes. It has a really engaging story.
In fanfiction, they often merge through the concept of interdimensional travel. For example, a portal might open that transports the Transformers to the Star Wars galaxy. Then, they start interacting with the different species and factions there.
Another way is when a character from one universe is sent to the other on a quest. Let's say Percy Jackson is sent to the Naruto world to retrieve an object that has somehow ended up there. His presence then starts to mix the two universes as he befriends the ninjas and they learn about his world.
Sometimes, it's a shared enemy that brings the two universes together. Maybe there's a super - powerful demon that is causing problems in both the world of the Winchesters and in Mystic Falls. So, the hunters from Supernatural and the characters from Vampire Diaries have to team up to fight it. This allows for a lot of interesting character interactions and new plot developments.
One way they can merge is through character crossovers. For example, a character from Game of Thrones like Jon Snow could be transported into the Warhammer universe. He might find himself in the middle of a battle between different factions in Warhammer, using his skills learned in Westeros to survive. His sense of honor and leadership could also have an impact on the Warhammer characters he encounters.
In many RWBY Space Marine fanfictions, they merge the two universes by having a portal or some sort of interdimensional rift. This allows the Space Marines to end up in the RWBY world. For example, in 'The Astartes of Remnant', a warp - like portal sucks in a group of Space Marines and drops them right into the middle of Vale.
In Young Justice love triangle fanfictions, common characters involved could be Robin (Dick Grayson), Artemis, and Wally West. Robin and Wally are best friends, and Artemis may find herself attracted to both of them in different ways. This creates a complex dynamic where jealousy and unspoken feelings start to play out.
Robin is often a central character. His intelligence and leadership skills are put to the test in a different way when he's de - aged. He has to rely on his wits even more as a younger version.
Another way is to have a character from one universe end up in the other, like Venom ending up in the Young Justice world. Then, through that character's eyes, we can explore the new universe and see how it clashes or meshes with their own. You could also introduce some sort of magic or technology that bridges the two universes, like a portal - creating device that was created by a mad scientist in both universes independently, but when activated, it links the two. This would bring in elements of mystery and discovery as the characters figure out how to use it and what it means for their respective worlds.