I've written from time to time about federated and multi-denominational Unitarian Universalist churches, in part because that's where you find many of the Christians in the UUA and also because they are an interesting polity situation that makes for illuminating case studies.
There aren't tons of them in the UUA -- most are in New England -- and they tend to be pretty stable. Federation partners include the United Church of Christ, the American Baptists, the United Methodists and the National Association of Congregational Christian Churches. Am I missing anyone?
In short, the difference between a federated church and multi-denominational community church is one of organization. In a two-way federation, there are three corporations: one for each denominationally-specific member church and a third corporation to manage the relationship. Each with its own board, budget (perhaps) and membership roll. A community church is a common entity -- one board, budget, incorporation etc. -- that holds its membership in two or more denominations. My read on federated churches is they became more palitable when ministers became scarce and more expensive -- so the federated church could share one -- because of general falling memberships. My gut instinct is that these relationships are more difficult when there is more than one building to maintain.
Last Wednesday's Worcester (Mass.) Telegram reports that a Unitarian-Congregational federation in Leicester, Mass. (no website) will end September 1. The church is functionally a family-sized church -- and knowing families -- I hesitate to suggest any reason for the de-federation than the one stated: a proposed move to merge the congregations into a community church.
I wish both churches well, and if you're a Unitarian Universalist minister in central Mass -- well, there's probably some supply preaching opening up soon for the Unitarian congregation of 10.