You can put two classes on one Portrait Auto Flow page and have them separated by group. This is different from having two classes on the same two page spread.
1. Make sure the page layout in the section you are using is set up so that all portraits will fit on the one page.
2. Move all the portraits to the page.
3. Change the order of the portraits so they are grouped the way you want.
4. You may want to make space on the page between the classes.