A OneText (literally "one text") is an agreement, a mutually desired output of two or more parties, in DocumentMode. It defines the consensus reached following a period of negotiation. The need for consensus on something is agree first, then the scope and nature of a OneText is determined, then negotiation begins. Negotiation can happen on a separate talk page, like in MixedMode (e.g., in MediaWiki there's a content-separated Talk NameSpace). The OneText is often authored by a third-party that tries to bring the two or more "sides" together. See NeutralPointOfView.
See also OneHyperText, a discussion on how to achieve one-text in hypermedia.
Some critical success factors for development of a OneText:
One way of reaching OneText is to use something like a DoubleWiki, where each party presents a text and they work on the differences. If you have an agreement, you may end up with the same text on both sides, in which case a DoubleWiki -like system isn't needed. If they don't agree, well you have a DoubleWiki - which is likely to happen if you lack some or all of the conditions above (which may often be the case for LandMine#TouchyIssues).
A DoubleWiki may be seen as a mid-way between OneText and complete anarchy. It's TwoTexts?, yes, but that know about each other, so (hopefully) more or less adapted to each other. For cases like drafting a constitution, a statement of princilples, a contract, etc. you only want OneText, but on other issues settling for a fair presentation of the different opinions present may be acceptable. Hmm, maybe a DoubleWiki should carry one section for the disagreeing, and one section for the common text - what both parties agree on. True, that looks a lot like the way wikis already work.