Whether to ask journal editor to review two papers simultaneously because the papers are mutually dependent on each other?
Honestly, it sounds a little like you were trying to Salami-slice your papers, and the reviews are showing this. If the design is not interesting / understandable / plausible without knowing the analysis, and the analysis is not understandable without the design, then the solution should be that you make this one larger, better paper, and not that you somehow make the reviewers aware of the other publication. One of the fundamental rules of scientific papers is that they are supposed to be self-contained - every paper should, by itself, be understandable and add something to the state of the art.

If the journal you are submitting to requires papers that are too short, you can always go for a journal with higher page limits.