If we truly wish to promote science or STEM education, then it would seem that the joining of resources and expertise from the communities of formal schooling and informal science education institutions or ISEIs (museums, aquariums, and the like) would be an important early step. Yet creating such connections between teacher and museum remain a challenge for both teachers and informal educators. This study employs a "communities of practice" lens (Wenger, 1998) to provide a deeper explanation for the challenges inherent in those programs and experiences (field trips, outreach programs, teacher professional development) where schools and informal science institutions interact to create unique learning opportunities. To better understand the expectations and perceived challenges of these boundary activities, questionnaires, focus groups and interviews were used to gather information from over 200 teachers and informal science institution staff in southern California. Several factors, identified by both groups, were found to influence these shared experiences, including issues of "authority", "capacity", "communication", and "complexity". By considering both educators' perspectives, we gain a clearer understanding of the practice of these communities and how such practices influence the activities at these boundaries. Furthermore, a closer look at "avid" users--those teachers who do successfully and regularly make use of ISEIs--reveals possible avenues for strengthening such boundary activities and potentially creating a more effective merging of learning and teaching resources.