Answer
Replace the semicolon-and-space delimiter the Receiver window displays with a plain comma and no space (User1,User2) — the Stream Message action's underlying user-list parsing expects a comma-delimited format, not the semicolon-space format the Receiver window happens to show when you select multiple users from its list, and editing the delimiter to a comma resolves the 'user does not exist' error for multi-recipient Stream Messages.