Final answer:
Two companies conspiring to set the same commission violates the antitrust law, as it constitutes collusion and is considered anti-competitive behavior, enforced against by the Department of Justice and the FTC.
Step-by-step explanation:
When two companies conspire to list at the same commission, they are likely violating antitrust law. In the United States, it is illegal for firms to engage in collusion, which is considered to be anti-competitive behavior. The act of collusion disrupts free market competition and, as established by the U.S. Department of Justice's Antitrust Division and the Federal Trade Commission (FTC), is illegitimate and subject to enforcement action. Such collusive behavior includes firms agreeing to fix prices or commissions, which is inherently aimed at extracting monopoly-size profits, thus harming consumers and other competitors.