In Victorious, Helen from Drake & Josh becomes the new principal of Hollywood Arts. She makes multiple references to her character from Drake & Josh, including having the same name and being the same child actress. She also mentions Crazy Steve, Jerry Trainor's character from Drake & Josh. This proves that the two shows exist in the same Nickelodeon universe.

Additionally, there is evidence that iCarly exists in the same universe as Victorious as there was a combined spin-off show, Sam & Cat.

Here's the issue: Miranda Cosgrove and Jerry Trainor play two different characters in Drake & Josh and iCarly. Since these two shows exist in the same Nickelodeon universe, that means Cosgrove is simultaneously Megan and Carly while Trainor is simultaneously Crazy Steve and Spencer.

While my original theory was that Crazy Steve kidnapped Megan and took her to Seattle to start their new lives as Spencer and Carly, I realized that Megan was much too smart for that. She would have anticipated someone having a grand scheme to abduct her, brainwash her, and give her a new identity and, as this plan would've taken an extended period of time, been able to develop her own plan to stop it.

My new theory is the opposite: Megan took Crazy Steve to Seattle to get the freedom she always desired. She constantly tried to get away from her brothers and parents through her pranks, so she found a legal adult who would be easy to trick. She convinced Crazy Steve somehow that he was actually her brother and got him to take her to Seattle as her legal guardian. They adopted their new names and created new lives for themselves as Spencer and Carly. Spencer's erratic art and constant confusion were symptoms of him coping with his new identity.

Or there are Miranda Cosgrove and Jerry Trainor doppelgangers in this universe, which is also plausible.