How do you avoid becoming the “middle man” between colleagues that don’t get along? Any advice for setting boundaries and establishing healthy communication?

Keep in mind that you should be a business professional first and a technology expert second. Utilize your business skills to facilitate communication within the organization. If you're professional, being the middle-man can be leveraged to your advantage and is a much better position in the organization than many in IT who struggle with communication in the business. Consider if it's really an opportunity in disguise. As a professional, you should always drive business communications towards collaborative teamwork in solving business problems, achieving business objectives, and driving value.

To avoid becoming the “middle man,” focus on enabling direct communication instead of relaying messages. When two colleagues don’t get along, encourage them to speak with each other directly and offer to support that conversation rather than act as a go-between. Set clear boundaries by stating your role is to support resolution, not manage their dynamic. Establish healthy communication by modeling transparency, staying neutral, and creating space for shared problem-solving, like facilitating a brief, structured meeting if tensions persist.

I tell them that I'm prepared to facilitate, but not mediate.  I'm willing to help the conversation move, but not drive agreement or good behavior; that's up to them.

Be honest and tell them that it is not up to you to make them talk and to have them solve issues, if there are issues they need to go to their manager.  Tell them that you are always willing to support but that they need to first sort out their differences of opinion.