Is it possible to limit editing of a Google Docs to only one user at the same time?
This is not supported directly. However, one trick is to invite several users, then in the advanced sharing dialog, for everyone but the one user you would like to edit, set the access level to "Can comment". For the user you want to allow to edit set access level to "Can edit". Later you can easily switch to letting another user edit through giving him "Can edit", etc.