Only one SCM is ever fully visible at a time. When the head turns to the right, the left SCM goes into full relief (the "tense rope"), while the right SCM squashes and folds slightly. If you sculpt both SCMs equally prominent in a turning head, you break the gesture.
If you cannot feel it on your own face, do not carve it. head+and+neck+anatomy+for+sculptors+pdf+exclusive
The trapezius muscle inserts along the spine of the scapula and the clavicle. However, there is a specific triangular depression—the Lesser Supraclavicular Fossa —just above the clavicle. This soft hollow is vital for realism. It separates the hard bone of the collar bone from the fleshy belly of the neck. If you bridge this hollow with clay, your sculpture will look like it has a "double chin" or a muscular spasm. Only one SCM is ever fully visible at a time