The MOST effective approach to the inability of two employees to work together would be...

The most effective approach to resolving interpersonal conflicts between employees is to speak directly to the individuals and to others to learn what the problems are (C). This allows for open communication, identification of the root causes of the conflict, and the opportunity to find a mutually agreeable solution. Simply ordering cooperation (A), threatening dismissal (B), or sending a general memo (D) are less likely to address the underlying issues effectively. (Fundamentals of Crew Leadership, Fourth Edition, NCCER, Section 2.4.2 Resolving Conflicts)