Each price sheet can only contain prints and products from one lab, as well as self-fulfilled products. You can create multiple price sheets to offer products from different labs, but only one price sheet can be associated with each gallery.
As an example, if you'd like your wedding prints to come from Lab A and your family portrait prints to be printed with Lab B, you can assign those price sheets to your galleries accordingly.
If there is a particular item you want to order from a different lab than the one your price sheet is set up for, you can always choose to self-fulfill that item, and then order it directly from the lab of your choice.