I did a quick search and couldn’t see any docs on this use case.
And it behaves the same way as `curl --header "" --header ""` in that it's additive:
$ curl -vK <(printf 'header = "alpha: 1"\nheader = "beta: 2"\n') httpbin.org/get
And it behaves the same way as `curl --header "" --header ""` in that it's additive: