When writing referee's report after paper revision, can I discuss the reports of other referees? I reviewed an article for a journal, suggesting some revisions. The authors made revisions and now the journal has asked me to do a second round of reviewing. As well as the revised manuscript, the journal also sent me the report written about the original submission by the other referee of the paper. 

I'm not a very experienced reviewer. This is the first time I've got to see someone else's report on a paper I've reviewed. When writing my report about the revised paper, am I allowed to discuss or respond to the other reviewer's comments? (When I've received referee reports on my own revised manuscripts, I've never seen referees discussing each other's opinions.)

In this case, reviewer 2 and myself wrote very similar reports, with reviewer 2 being a bit harsher. There is however one point in reviewer 2's report that I'd like to reply to directly. He/she criticises the authors' results X because they don't also show Y. Showing Y at the same time as X has been a long-standing goal in our field. But nobody has ever succeeded, and recent studies suggest that probably X doesn't imply Y after all. The authors do a poor job of defending themselves. There was no mention of Y in the original submission and I didn't mention it in my report either.

In general, is it okay to write in a referee report "The other referee says... but actually...". Or should it be in a separate comment to the editor? Or should I try to rephrase so that I'm responding to the authors instead of the other referee (not easy in this case).

Maybe I'm overthinking this, but it seems to me (i) I'm supposed to be reviewing the paper, not reviewing the other referee, and (ii) if this should go to yet another round of revision, it shouldn't develop into an argument between the 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.