An XXY Zygote splitting into two distinct lines of XX and XY, related to Klinefelter syndrome where the mosaic is XXY/XY. That's what they think happened in Foekje Dillema's case, where researchers went and
located genetic material way after the fact and found that she was an almost 50/50 split, right down to the skin cells.
There's also the potential for 2 zygotes aggregating into a single embryo, essentially fraternal twins glued together, but technically that's a chimera, like you said. It doesn't produce the 50/50 split that researchers saw in Dillema's case.
I note the even in the article I quoted, the researchers say
Foekje was registered and raised as a girl/woman, and there was never any indication that she operated as anything else. She never acquiesced to an inspection of her body, so it's possible that she had ambiguous genitalia, but most people with ovotesticular DSD are raised female. People with DSDs are the one case where there is a little bit of messiness when it comes to how to identify them, but it doesn't come from the same place as trans nonsense. It's essentially coopting the DSD experience.