M Visa Question

Hi,
I have a single entry, 6 months M which I got from the Chinese embassy in Tokyo. (I am an American with a Japanese residence card) I would like to travel outside of China and return during my stay here. What do I have to do in order to do that? Do I have to leave China and apply again at the embassy in Tokyo for a multiple entry one? I visited China previously on an L so I have 2 Chinese ones on my passport (including my current one). Someone mentioned to me that having 2 previous stamps were a requirement to get a multiple entry pass.

Thanks