Now The Other Shoulder Is Giving Out
A decision on liability is one for the insurance company to make, not for the treating doctor.
Of course if the first medical certificate records an injury to both shoulders, then the claim is much stronger. I cannot emphasize how important it is to document early on all injuries sustained. Once documented, they should be fully investigated.
It certainly makes sense to me to say that if there is a temporary overload of one joint due to immobilization of another joint, then the two are interrelated. In this situation I have frequently seen insurance companies cover the cost of investigation and initial treatment on a “without liability” basis.
Obviously if there is a significant new problem that might require surgical treatment, then a different level of assessment may apply.
Another factor in the determination of these types of claims is the timeline. If the symptoms developed at the time of immobilization then there is a connection. But if the symptoms developed twelve months later it is harder to make a supportive case.
In my experience, insurance companies are generally very supportive in relation to the short term treatment for secondary physical conditions and we seldom experience significant conflict in this area.