Under the Model Rules, we know that you can't sue a current client, even in an unrelated matter. On the other end of the spectrum, we also know that there's nothing improper about representing two parties, even though they compete economically. Both propositions are well-known.
In between, there is obviously a range of behavior that, at some point, crosses the line from permissible to impermissible (absent consent), and, ultimatley, to impermissible even with consent.
Suppose Firm 1 is hired by Client A to bring suit against some potential infringers. Client A discloses a list of 5 possible infringers to Firm 1. One client on that list is another client of Firm 1's, Client B. Firm 1 probably should tell Client A that Client B is a client (AWD got sued several years ago for failing to do so, as this could constitute an undisclosed material limitation on the lawyer's ability to represent Client A). Obviously, the firm can't sue its own client. Can it ethically represent Client A against non-clients, when as a result the firm might be creating rulings (e.g., Markman decisions) that will be used against Client B?
Thoughts? My own view is that this creates some risk, and there is some caselaw on this "indirect adversity" issue, holding that essentially a lawyer can't help one client "make a case" against another client, even if there is no direct adversity. None of that authority, though, is in the patent litigation context, and I'd have to ponder it more than I have to have a real answer. But what do you think? Is it consentable?