I've been thinking about this the whole day. At first I thought it was an easy fix, given my dragon's bones have a tensile strength of 3-6.5 GPa and an elastic modulus of 50-140 GPa, depending on the reinforcing fiber's orientation. For limpets, this fiber is goethite, for my dragons, I take the risk of using carbon nanotubes.
The problem here is that the extra layer, I put on top of the original pectoralis major, has to be the same length as the original (that is the requirement of circumventing the square-cube law), and needs a suitable attachment area and good leverage. Plus, it seems that the extra layer would be in the way of the downstroke.
Of course, dragon bones don't have to look like anything you'd expect find in nature. The creation myth and thus, creationism, is an integral part of this world. Though even the gods have to obey the laws of physics.
Before you say, I know it'd be easier to just increase the length of the ribcage and redesign the humerus to provide more attachment area, but I hate limiting myself.
How should the bones be redesigned to allow adding flight muscles to them (with some exaggeration) ad infinitum?
Note: It's kinda bad that most of these pictures don't show where the muscle ends and the tendon begins.