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In a back-to-back water closet scenario, the minimum horizontal developed length required between the water closet outlet and the double sanitary tee connection is set to ensure proper drainage and venting. The 18-inch distance is crucial as it helps prevent siphoning between the two fixtures and maintains effective wastewater flow.

By adhering to this minimum length, the plumbing system minimizes the risk of clogs and ensures that each toilet can operate independently without affecting the other. Proper spacing also allows for necessary clearances and reduces the potential for unpleasant odors from one toilet affecting the other. This specification is based on plumbing code requirements that focus on maintaining optimal functioning and safety in residential plumbing systems.