Your wording is quite specific here. In this scenario a breach of professional ethics would only be called into question if Project A and Project B were going on simultaneously inside the firm. The above examples illustrate an attempt to separate project activity in side the firm, however a judge and the attorneys could easily find fault in such a case as presented. It is quite common for a firm who is known for a certain particular kind of design work (i.e housewares) to wrestle with this kind of ethical dilemma on an on going basis.
I’ve witnessed both strict adherence to this kind of conflict of interest scenario, as well as interns making shrouds out of paper to hide the competitors product in the studio when the "other"client is there for a visit. One of the realities in many design studios is that the showroom/project wall is open and is used to show who and what kind of projects the firm has worked on in the past. This usually serves as positive when a new “blender” client sees that you have done work for another blender client in the past.
Consumer electronics, footwear and automotive are among the most sensitive and dubious in this area of professional ethics. The lawsuits that name firms and designers as defendants can be quite large.