Whether to ask journal editor to review two papers simultaneously because the papers are mutually dependent on each other? Having read this earlier question I find myself in a similar, but different situation and would like to ask your advice.

My colleagues and I have created a new medical imaging system, and have performed a series of standard tests on this system. We want to publish two papers - one describing in detail the design of the system including some key performance parameters, and another describing in more detail all performance (with a brief summary of the design). Ad the device is now commercially available, there is significant interest in the community to have more details in peer-reviewed format.

Both papers were submitted to the same journal, and were reviewed by different reviewers. Comments were along the lines of "this would be great if it contained [more of what is in the other paper]". In other words - neither paper is currently accepted because it is missing information that is in the sister paper.

Unfortunately the journal in question has a hard limit on length (5000 words including reference) and figure/table count; combining the two into a single paper would seriously hamper our ability to communicate the details that would be of interest to our target audience.

So here is the question: would it be considered acceptable to contact the editor, explain the situation, and request that the two papers be published side by side in the same journal? It would largely address the reviewers' concerns. The alternative would be to request an exemption from the word / figure count.

I don't know whether either of these things is commonly done. Your advice is appreciated.
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.