If two lavatories are installed back to back in an 8 1/2 inch wall _________________.

In the scenario where two lavatories are installed back to back in an 8 1/2 inch wall, the correct approach is to connect each fixture to a separate waste. This is essential because each lavatory generates a significant amount of wastewater, and providing individual connections helps ensure adequate drainage and venting.

By connecting each lavatory to a separate waste line, you prevent potential backflow issues and maintain efficient operation. This also allows for easier maintenance and troubleshooting if one fixture develops a blockage or other plumbing issues, as each system is independent.

In contrast, connecting both fixtures to a single 1 1/2 sanitary cross may not provide sufficient drainage capacity due to the combined flow, potentially leading to clogs. Additionally, the other option regarding a single 1/2 inch sanitary cross is not appropriate because the diameter is inadequate to handle the combined waste of both lavatories. Therefore, separate connections are the best practice in this installation scenario.