no, only one primary key column in the table, not 2.
in the replyID column, you will store the value from the primary key column of the comment it is a reply to.
lets say you have a comment record where the comment_id column is "22"
to create a reply to that comment, in the new comment record that is created, the replyId column will get the value "22"
the reply ID column is used to tell it which comment the reply relates to.