Toner does not adhere well to smooth surfaces such as litho inks, coatings. Toner needs a surface with a little "tooth" to bite in and adhere. The best advice is not to design a job with toner overprinting litho ink. If you are going to imprint toner over litho ink you may want to print the litho ink as a light screen (20% - lighter the better) to allow some of the paper surface to be available to bond the toner. Consider a third pass through a coater (UV) to seal the litho and toner. If the job requires over printing the litho ink, the areas that over print should all be printed digital. Be aware that toner printed on litho inks is likely not to fare well in abrasive environments such as going through mail sorting equipment.