Are sternocleidomastoid muscle syndrome and tmjd related? Does one problem cause a flare up of the other? Symptoms of both seem similar.

Often related. The muscles of the head and neck area are often involved in temporomandibular joint disorder. Muscle spasm is a common finding and many different sets of muscles can become tender/spasmotic when the condition flares. Muscle spasm is the body's mechanism to prevent movement of a painful joint.
Confusion. There is no sternocleido musclevsyndrome. It is probably myofascial pain disorder or myofascial pain syndrome which in fact is a sub division of temporomandibular disorders or TMJ dysfunction.
TMJ. Yes through fascial connection triggers in one area may aggravate the other. This is not always the case.