Working on a single project doesn’t limit you from using multiple designs. You can create and use many designs for different products or categories. Designs work in sequence, meaning their order does matter.
By default, the design placed on top of the list will be your default and will apply to all products (assumed no additional filters are applied on it for dates, IDs, promo price or categories).
To reorder your designs, simply use the drag & drop function next to the ON/OFF toggle. Once you have changed the order, go to the "Settings" tab and hit "Update Now".
NOTE: If any of your products is assigned with multiple designs at the same time, the one placed on top will be prioritized above all others. In order to apply multiple designs to a single product, you must create a separate project.