Did you might happen by chance to mean that you had a 50 lb bag of starter grower feed? Because if you have a 50 lb bag of layer pellets then you don't have to add anything to it have layer feed. If you are concerned about the 2 birds that are not laying yet eating the layer feed then just wait to add oyster shell as a supplement until they all are laying. When you do offer oyster shell don't mix it with their layer pellets, offer it to them in a separate container and they will eat it when they need it, they know when they need the extra calcium.
Basically what I'm saying is Layer Pellets are Layer Feed. I think you mixed up you words or thoughts somehow, please clarify.