The title generally bestowed when multiple people head the concom for a science fiction convention, sharing the traditional duties of the chairman. Traditionally, anything that the convention does is the chair's responsibility since responsibility flows upwards.
There are several reasons why a co-chairmanship arrangement might be used:
(1) When there is no one who is fully qualified to chair a convention, the idea of finding two people who each fall short of ideal, but whose strengths are complementary and having them co-chair in the expectation that their weaknesses will cancel and their strengths combine. It usually doesn't work out that way, since it is more common for the individual weaknesses to add up than it is for the individual strengths to add up.
One common problem with co-chairmen is that the committee usually quickly discovers that if Daddy says "no", I can always ask Mommy: unless the co-chairmen coordinate very closely and have complete trust in each others' decisions, this will work. Since that coordination and trust is difficult to achieve and requires high management skills (and if either of the co-chairmen had those skills, they probably would have been appointed as sole chairman), it is common for co-chairmen to divide up responsibility. This cuts down on the ask Mommy/ask Daddy problem, but risks having a dual convention structure where the two sections don't coordinate very well. And, since the problem that the co-chairmanship was created to solve was that neither co-chairman was really up to working on their own, this arrangement usually struggles to achieve mediocrity.
(2) When a group's politics means that there are two competent candidates and neither is willing to let the other chair, a co-chairmanship is sometimes tried. It takes a high degree of maturity on both parts and usually works about as well as can be expected.
(2a) Sometimes the two individuals come from two groups which are running the convention in coalition. This works about as well #2, above, with the added strain of culture clashes.
In both cases (1) and (2), it's not uncommon for one of the co-chairmen to become the dominant decision-maker. In case (1), this is usually unfortunate, but in case (2) where both co-chairmen are competent, it can work well as long as ego-bruising is avoided.
(3) Usually, whoever runs the bid committee expects to chair the resulting convention. But the skill-set to run a good bid is not much like the skill-set needed o run a good Worldcon. Appointing a co-chairman (who will actually run things) as a face-saving device can be successful. But this can too-easily devolved back into a split responsibility situation.
(4) Couples can often function successfully as co-chairman as long as their marriage is strong enough, since this eliminates most of the Mommy/Daddy and communications problems.
(5) A candidate relatively new to fandom may get paired with an older, more respected, fan for the sake of assuring voters.
And sometimes any of these possibilities work out. But don't bet on it.
There are three main reasons who split management usually fails:
- The aforementioned Mommy/Daddy problem
- The aforementioned problem where a co-chairmanship allows weak managers to get in over their heads
- Lack of communications. Any one-shot convention (like Worldcon) requires continuous, effective communications between the levels of the con's management. Experience has shown that when two (or more) people act as joint leadership, and genuinely try to make it work, they spend so much effort communicating with each other that they neglect communication with the rest of the committee. They has out policy between themselves and then tell the rest of the committee -- this is not a good way to retain competent staff nor is it a good way to make use of the expertise elsewhere in the committee.
(But innovative committees are always finding new and different ways to fail.)