Musíte se JOIN
users
tabulky dvakrát a přiřaďte jim různé identifikátory (aka aliasy ) na každém JOIN
v rámci vašeho 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