Abstract: A tandem dual-head lithographic duplicator is equipped for operation in alternative modes, i.e., either for duplexing by printing on opposite faces of the copy sheets in sequence, or for simplexing. In the simplexing mode the printing of the copy sheets is carried out on one head simultaneously with the cleanup and preparation operations on the other head, with a sequence of printing jobs being performed alternately on one printing head and then on the other while, in each case, the non-printing head is being prepared. Accordingly, substantially continuous simplexing operation can be achieved except in the case of very short printing runs.