Assuming the following scenario:
Person A is a recognized senior researcher, and Person B is an enthusiastic junior researcher. Both of them have been working more or less equally on a research project and are interested in submit the results of the project to a particular venue. Based on the report written by Person B, Person A has written a paper (PA) to be submitted to the venue. Person B doesn’t like the style and some contents of PA, so he makes comments to Person A, who them modifies PA. Person B still doesn’t like PA, so writes a separate paper PB. Since PA and PB haven’t been published, there are citations between them.
At this point, Person A submits PA, which list authors as "Person A & Person B", and Person B submits PB, which list authors as "Person B & Person A."
- Is it appropriate to submit PA and PB?
- What would be your impression if you were part of the venue’s committee?
- As reviewer, what would be your impression about reviewing two different papers about the same work for the same venue?
- Any additional consideration?
Most venues forbid simultaneous submissions, or overlapping submissions. For instance, in the call for papers of ACM CCS 2012:
Submitted papers must not substantially overlap papers that have been published or that are simultaneously submitted to a journal, conference or workshop. Simultaneous submission of the same work is not allowed. Note that submitted papers cannot be withdrawn from the process after the first phase reviews are received by authors.
So, in order to answer your question, the main problem is determining the overlap between PA and PB. If it's substantial (only some elements of style change, minor comments), then it should be forbidden.
On the other hand, if the papers are different enough (for instance, PA presents a more theoretical approach and PB a more practical description, with a case study), then there is no problem. That being said, both papers must be self-contained, and cannot rely on the acceptance of the other, unless they are both also published as technical reports (or equivalent).
To make things explicit, if I were a reviewer of a member of the PC, and if I were to think that the overlap between the two papers is substantial, I would recommend rejecting both.