I am halfway through a PhD in History in the UK, in a highly unusual subfield. As my competences are quite rare, I have already been asked to help out with, and be a co-author of, two different papers with postdocs/professors in other areas of the Humanities. The results are interdisciplinary articles that I am quite happy with. I am also writing an additional article with a fellow PhD student (I will be the first author there). As I am only now finishing up my first solo paper, I am concerned that having more co-authored papers (3) than solo ones (1) will hurt my CV when I apply for postdocs/jobs. Will institutions look down on me for co-authoring more papers than the norm in the Humanities?
In the UK, up until last year, hiring for lectureships was all about the Research Excellence Framework (REF). Going forward, hiring will likely be all about the REF. We just made two hires in my department and the keys questions were what did the applicants look like for the last REF and what might they look like for the next REF.
Co-authorship in my field is the norm and for the last REF where on the authorship list you were did not matter. I am pretty sure this was true across all assessment units, but you should check with someone in your department. For REF 2014, the same paper could be returned multiple times if the authors were spread across multiple universities (and possibly even multiple units of assessment). For REF 2020, we do not know what the rules will be. Potentially, they will attempt to assign partial credit to publications or only allow a paper to be returned once, but this would be hugely messy.
If REF 2020 devalues co-authored papers in History, and by 2020 you won't have N (last time N was 4, but could be reduced for new lecturers) single authored publications, then the joint publications will be looked at unfavourably.