I think my problem is with the interfaces. When making the named selections, I create an interface between the faces for air-to-bag interface, then I make another for the faces for bag-to-blood interface and they end up being the same exact interface. I get an error stating there are overlapping named selections, and when I highlight each individual named selection they are the same thing. For example, if I hide the geometry for the blood inside the bag and then highlight the named selection for the bag-to-blood interface, it shows the faces from the air-to-bag interface. Essentially, it seems to be acting like the bag is not there.
Could this be an error in my geometry setup? I created the bag by copy-and-pasting the faces of the blood volume and giving those surfaces a shell thickness during meshing. I can highlight and see there are different surfaces for the bag and the blood, but when I try to apply a named selection it seems to mess things up.