[BUG] git diff-tree --stdin doesn't accept two trees

I'm trying to use diff-tree --stdin to diff several trees in one go. But I just get error messages when I feed it two space-separated trees (one commit works fine):

  $ echo $(git rev-parse HEAD^{tree}) $(git rev-parse HEAD^^{tree}) | git diff-tree -p --stdin
  error: Object 7bfd9971f77438858e412be0219ec78afb3ca46f not a commit

This is at odds with the documentation:

  --stdin::
        When '--stdin' is specified, the command does not take
        <tree-ish> arguments from the command line.  Instead, it
        reads either one <commit> or a pair of <tree-ish>
        separated with a single space from its standard input.

I tried reading the code to figure out what's wrong, and as far as I can tell the code to do this is there, but seems to be protected by logic that aborts everything unless the whole input line is a valid commit. Or maybe I'm just confused ...