On October 10, Steve Garvey said he would run for the U.S. Senate in California as a Republican. See this story. He is famous for his sports accomplishments in baseball and also football.
If two prominent Republicans were to run for that Senate seat, it is conceivable, though unlikely, that the top two vote-getters in the March 2024 primary would be two Republicans. There are three prominent Democrats running for the seat, all of them members of the U.S. House. More well-known Democrats might also emerge, including possibly the appointed incumbent Democratic Senator, Laphonza Butler.
A November 2024 California ballot with only two Republicans listed, and with no-write space, would certainly upset the top-two system. Thanks to Thomas Jones for the link.