conf/_admin.php:

/**
 * Cross posting
 *
 * Possible values:
 *   - 0 if you want users to post to a single category only
 *   - 1 if you want to be able to cross-post among multiple categories
 *   - 2 if you want to be able to cross-post among multiple blogs/categories
 *   - 3 if you want to be able to change main cat among blogs (which will move the
 *       posts from one blog to another; use with caution)
 *
 * @global int $allow_cross_posting
 */
$allow_cross_posting = 1;

tag for search: crosspost crossposting