I think you want something like this:
#set text(font: ("Songti SC", (name: "Libertinus Serif", covers: "latin-in-cjk")))
That is, if your main document language is Chinese.
Otherwise, I think this should work:
#set text(font: ("Libertinus Serif", "Songti SC"), fallback: false)
The fallback: true will ensure that no other font is used, but this could potentially mean that some characters will not be displayed.