I do methodological research in Field A and am aware of the the related methodologies of Field B but have not used them. I was asked to review an article from Field B, which I initially declined due to lack of expertise. The editor asked me to reconsider because the article makes some claims about Field A and he wanted comments specifically from that perspective. I agreed to review.
I am reviewing the article and the editor was right to be concerned. The paper is otherwise excellent, however the authors are claiming that what they are doing in Field B is actually Field A (and using terminology from Field A but have ignored substantial literature). I am fine to write the review. However, I would really like to make a comment recommending that any mention of Field A should be removed from the paper (it would be fine without it) and noting that the overlap is an interesting question in its own right that should not be dealt with from the perspective of just one of the fields within an empirical paper about a specific topic.
I actually think that a specific methodological paper about the overlap and differences would be very worthwhile, coauthored from both fields and preferably written as the outcome of some sort of discussion at a workshop where each field trained the other. While the review is blind, I strongly suspect that the authors are from the institution that would be the most appropriate to develop such a workshop and paper. However, I am already aware of a researcher in my field who is drafting a paper (from the perspective of Field A, with no involvement of authors from Field B).
If I make the comment about interesting question in its own right, am I suggesting the separate paper? If so, is that a problem given that I know someone is working on something similar? Can I suggest to the person working on it that perhaps he should involve authors from the other field? Can I suggest a workshop to both the person working on it and the authors of the paper I am reviewing?