How do informal learning organizations work with schools as part of a broader educational ecology? We examined this question through a comparative case study of two collaborative efforts whereby informal arts education organizations, a children's museum and a community-based organization, worked with an urban school district to redefine the provision of educational services for children and youth. Grounded conceptually in organizational theory, our study identified factors that enable and constrain collaboration across the formal-informal divide. We argue that examining the dynamics of cross-sector collaboration as occurring within a regional ecology of diverse learning organizations and broader institutional context provides insights into the outcomes of joint work. Our findings have implications for designing collaborations between schools and informal organizations that contribute to their respective strength, as well as the broader regional educational ecology in which they reside.