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!