Conflicts of interest often arise in legal cases and their ramifications can be far-reaching. So many conditions bind attorneys and clients and they both must be careful and not create a legal conflict of interest. The American Bar Association Model Code of Professional Responsibility and Model Rules of Professional Conduct cover differing areas of situations.
Simultaneous representation is one conflict of interest.
This one has little room for interpretation and is pretty well specified in what is not allowed. An attorney may not represent two clients who are adversaries in a case. The reasons are obvious in that no attorney could rationally represent both sides and do an adequate and fair job to either. Another situation that can arise with this is an attorney representing opposing parties of differing cases. As long as both clients are advised of the situation and feel the attorney can do an adequate job of representation, then it is permissible.