Z Visa

If applying for a Z Visa (allowing an applicant to work at a trade exhibition in China) and being granted a maximum stay of 12 days - if the visa holder wished to extend their stay in China for additional touristic purposes, could this be down via an additional tourist visa in their passport? Or can you only hold one type of Chinese visa at any one time? Or might the visa holder have to depart China before the expiry of their Z Visa and then re-enter China on a tourist visa?