Another way is through a magical or technological experiment gone wrong. In certain RWBY Space Marine fanfictions, a RWBY character might be experimenting with some sort of new Dust - based technology that accidentally creates a link to the Space Marine universe. Or, a Space Marine ship's warp drive malfunctions and they end up in Remnant. Once they are in the same world, the two groups have to figure out how to coexist and work together, which is how the two universes are merged in the story.
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.
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 fanfictions, the two universes merge through magical portals. For example, in some stories, a powerful spell in the Winx Club world accidentally creates a portal to the Pokemon world, allowing the characters to cross over.
In some fanfictions, the merge happens when a powerful artifact from either universe is activated. This could be a Kamen Rider's belt or a powerful relic from Highschool DxD. When this happens, it creates a rift that pulls elements from one universe into the other. The characters then have to deal with the new and unexpected situations. For instance, the devils in DxD might be intrigued by the Kamen Rider technology and try to harness it for their own purposes.
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.
In many fanfictions, magic is the key to merging the two universes. Fem Harry Potter's world of magic can act as a bridge. For example, a powerful spell might accidentally open a portal between the wizarding world and Metropolis, allowing the two to interact.
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.
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.
Some fanfictions merge the universes by having a shared event. Like in 'The Great Crossover Event', there is an event that draws characters from both the Harry Potter universe and the Yu - Gi - Oh universe together. Fem Harry Potter uses her magical skills, and she has to learn about the dueling system in Yu - Gi - Oh to participate. It's all about how these two different types of magic and combat styles interact with each other.
One way could be through magic. In the world of Harry Potter, Tom Riddle is a powerful wizard. In Game of Thrones, there is also magic, albeit different. Riddle could perhaps use his dark magic to cross over into the Game of Thrones universe. Maybe he'd be drawn to the power struggles there, similar to his quest for power in the wizarding world.