Coaching for a Difficult HR Conversation

I'm the Executive Director of a small nonprofit (5 staff) and co-lead the organization alongside our Founder/Artistic Director. We're peers, both reporting directly to the Board, with no one above either of us operationally. HR responsibilities fall under my role, but we have no dedicated HR staff or support.

I'm preparing for a difficult conversation with my co-leader about a pattern of conduct affecting shared staff (dismissiveness, sharp reactions to clarifying questions, after-the-fact criticism of decisions staff made in good faith after being told to move forward independently). Because we're structural peers, I have no authority to manage or discipline him, and I'm also the person who'd normally handle HR concerns, which creates a conflict of interest I don't have a clean way to resolve internally.

During our 60 minutes, I'd like to cover:

How to frame and structure this conversation given the peer (not supervisor) dynamic
What documentation I should keep before and after, given I may need to involve the Board
Whether a third party should be present, and if so, who that should reasonably be in an org this small
Specific language to avoid, especially given I'm both a party to this and technically the HR function
Whether/how to recommend a governance fix to the Board (e.g., designating a committee as the neutral path for co-leadership or personnel concerns involving either of us) and how to raise that proactively rather than reactively
Any compliance considerations specific to nonprofit co-leadership structures with dual reporting lines