You’re right, that the line can blur and when I read the question at first, I thought, ‘they’re not really that similar’ but then I thought about it, and there are a lot of similarities - style-wise, especially with a lot of the ‘old school’ scene kids. Take Lexi Lush for example. She was just on the line of both sides. She had scene hair, but dressed like a kandii kid. She doesn’t dress like that anymore, but there was a time when she’d wear so many beaded bracelets it’d come up to her elbows, and wear all the neon and such.
I don’t think people can only fit into one subculture or fashion, but I see where you’re coming from with your question, so I’d basically sum it up as: scene is usually, primarily, about the hair and make-up. Also, scene fashion takes more inspiration from 80s metal, whereas kandi fashion seems to be more neon, and 90s inspired. Scene kids wore a lot of neon too, but it was mostly pink neon, and kandi seems to just go for anything bright at all.
Sorry this got so long-winded, and I hope it didn’t come across as sounding rude.
I hope I helped :)