A lawyer cannot competently represent two people in the same case when the two people have adverse interests for the same reason why the world never saw heavy-weight boxing champion Cassius Clay take on the greatest boxer of all time, Muhammed Ali. If he had won, he would have lost. If he had lost, he would have won. (But it might have been fun to watch!)
But seriously, if a lawyer represents two people with conflicting interests, he’s in the wrong but does that mean he’s committed legal malpractice? No. Even if your lawyer has done something wrong, you must still show that his or her error caused you damages.
Rick Donato DeMartini appears to have been unable to get a lawyer to take his legal malpractice case against the Marshall, Minnesota law firm of Stoneberg Giles & Stroup but he managed to get over the tall hurdle of getting an expert opinion offering an opinion of professional malpractice. Still, it was insufficient to make a claim; the court held that the opinion failed to show how that alleged attorney error caused plaintiff’s financial harm and the case was dismissed by the trial court and the dismissal affirmed by the Minnesota Court of Appeals.