When writing referee's report after paper revision, can I discuss the reports of other referees?
Review are not an appropriate location for a direct argument between the referees.

There are, however, appropriate ways to respond to what you see as a mistake by the other referee.


First, you can make a clear statement of the positive value in the paper, implicitly contrasting with the other referee.  For example, in this case, you can simply talk about how significant and publishable the authors work on X is.  If they now mention Y in their paper, you can note that you are not concerned about it, for the reasons that you stated above.  In your open review, keep it purely about the paper.
Second, however, every review form comes with a place for confidential comments to the editor.  This is the appropriate place for you to respond directly to the comments of the other referee, saying things like "I think Reviewer #2 was too harsh, because...", since it is the editor's job to synthesize the disagreements of reviewers into a decision on the paper.


Finally, you probably don't have to sweat it too hard: since the paper wasn't rejected after the first round, that usually means the editor thinks it has the potential for publication, with appropriate revision.