Unfair co-first authorship
Maybe a workaround would be to explicitly assign credit in the paper. This seems to be used in some disciplines and I think it makes a lot of sense, especially in an interdisciplinary project where customs about authors order might differ. 

It simply consists in a footnote at the beginning of the paper or a section at the end where all the authors are listed and their contribution is summarized, e.g.:


A conducted experiment X
B collected Z for experiments X and Y, prepared Z and conducted experiment Y  
C proposed the approach and designed experiments X and Z
D proofread the paper
...


In my domain I'm not very familiar with this usage, but I've seen it in a couple of papers (in medicine as far as I remember). 

This could be a way for you to avoid a serious conflict in the project, while making clear to any reader which author did what.