I am a referee for two papers by the same author: can I acknowledge in the second report (the first was almost ready when I was invited to referee the second one) my report to the first paper?
To give more details: I am suggesting acceptance to both papers, but I think the second one reexplains too much of what is developed in the first paper (with proper citation, but to a slightly different version in preprint), and I want to be able to tell the author they should simply cite the other work and move on to the new results. Furthermore, both papers make some odd choices about presentation, and it would be easier for me to simply point to the other report than to explain again in great detail why e.g. an alternative definition of some concept would make the work much easier to follow.
This is a single-blind peer review, so I know who the author is, but they don't know who I am. I don't see how doing this could be a problem (both in terms of ethics and etiquette), as they won't be able to tell who I am just from knowing I am a referee for both papers, but it seemed wiser to me to ask, rather than assume it is perfectly OK.
Edit: the papers were subjected to different journals, and are being handled by different editors. So it was just by coincidence that both ended up with me as a referee; but not a big coincidence because the field is rather small...
Edit 2: I have accepted Buffy's answer because of the general advice on how to proceed: contact the editors, and keep in mind that the first paper may not get accepted and that the author may get confused. But, in my specific case, I am following more or less quarague's suggestions: I am writing the two reports more or less independently, and referencing the preprint when talking about the first paper on the second report, but I will not make great efforts to hide that the two reports have been written by the same person (I will not explicitly mention it, but it will be clear from the similarities between the comments on both reports).