Musisz JOIN do users tabeli dwukrotnie i nadaj im różne identyfikatory (inaczej aliasy ) na każdym JOIN w Twoim SQL.
SELECT
comment,
userFrom.username AS commentFrom,
userTo.username AS commentTo
FROM comments
JOIN users AS userFrom ON userFrom.ID = comment.commentFrom
JOIN users AS userTo ON userTo.ID = comment.commentTo
