I don't know whether that use of the em-dash is grammatically correct, but I've seen enough native English writers use it like that. One example is Philip K Dick.
Perhaps you have—or perhaps you've seen this construction instead, where (despite also using "or") the phrase on the other side of the dash is properly parenthetical and has its own subject.
Perhaps you have—or perhaps you've seen this construction instead, where (despite also using "or") the phrase on the other side of the dash is properly parenthetical and has its own subject.