Facilitation is a process of helping groups work cooperatively and effectively. Facilitation can be particularly helpful for individuals who, based on a certain level of insecurity or inexperience, tend to lurk rather than participate. At the same time, in peeragogy, a facilitator isn't necessarily an "authority." Rather, facilitation work is done in service to the group and the group dialogue and process. For example, a facilitator may simply "hold space" for the group by setting up a meeting or a regular series of discussions.
Co-facilitating in peer-to-peer learning
Co-facilitation can be found in collaborations between two or more people who need each other to complete a task, for example, learn about a given subject, author a technical report, solve a problem, or conduct research. Dee Fink writes that "in this process, there has to be some kind of change in the learner. No change, no learning" . Significant learning requires that there be some kind of lasting change that is important in terms of the learner's life; in peeragogy, one way to measure the effectiveness of co-facilitation is to look for a change in the peer group.
Co-facilitation roles can be found in groups/teams like basketball, health, Alcoholics Anonymous, spiritual groups, etc. For example, self-help groups are composed of people who gather to share common problems and experiences associated with a particular problem, condition, illness, or personal circumstance. There are some further commonalities across different settings. Commenting on the work of Carl Rogers:
Godfrey Barrett-Lennard: The educational situation which most effectively promotes significant learning is one in which (1) threat to the self of the learner is reduced a minimum, and (2) differentiated perception of the field of experience is facilitated. 
Part of the facilitator's role is to create a safe place for learning to take place; but they should also challenge the participants.
John Heron: Too much hierarchical control, and participants become passive and dependent or hostile and resistant. They wane in self-direction, which is the core of all learning. Too much cooperative guidance may degenerate into a subtle kind of nurturing oppression, and may deny the group the benefits of totally autonomous learning. Too much autonomy for participants and laissez-faire on your part, and they may wallow in ignorance, misconception, and chaos. 
Co-facilitating discussion forums
If peers are preparing a forum discussion, here are some ideas from "The Community Tool Box", that can be helpful as guidelines:
Explain the importance of collaborative group work and make it a requirement.
Establish how you will communicate in the forum.
Be aware of mutual blind spots in facilitating and observing others.
Watch out for different rhythms of intervention.
Co-facilitating wiki workflows
A good place to begin for any group of co-facilitators working with a wiki are Wikipedia's famous "5 Pillars."
Wikipedia is an encyclopedia.
Wikipedia writes articles from a neutral point-of-view.
Wikipedia is free content that anyone can edit, use, modify, and distribute.
Editors should interact with each other in a respectful and civil manner.
Wikipedia does not have firm rules.
Co-facilitating live sessions
Learning experiences in live sessions are described in the article Learning Re-imagined: Participatory, Peer, Global, Online by Howard Rheingold, and many of these points are revisited in the handbook section on real-time tools. But we want to emphasize one point here:
Howard Rheingold: Remember you came together with your peers to accomplish something, not to discuss an agenda or play with online tools; keep everything as easily accessible as possible to ensure you realize your goals.
Fink, L. D (2003). Creating significant learning experiences: An integrated approach to designing college courses. John Wiley & Sons.
Barrett-Lennard, G. T. (1998). Carl Rogers' Helping System: Journey & Substance. Sage.
Heron, J. (1999). The complete facilitator's handbook. London: Kogan Page.