My inclination is to say that it depends on who will be the lead responder for each plan and if there are different people responsible for responding. If there are limited employees available and responsible for carrying out the disaster plan, it may make sense to combine them. However, there is an abundance of information for both small and large museums regarding emergency disaster planning and you may be able to find more satisfactory information there.
openknowledge.worldbank.org/handle/10986/10125 Disaster Preparedness for Cultural Heritage
http://www.getty.edu/conservation/publications_resources/pdf_publications/pdf/emergency_plan.pdf Building an Emergency PlanA Guide for Museums and other Cultural Institutions.