Programming industrial robots is difficult and expensive. Although recent work has made substantial progress in making it accessible to a wider range of users, it is often limited to simple programs and its usability remains untested in practice. In this article, we introduce Duplo, a block-based programming environment that allows end-users to program two-armed robots and solve tasks that require coordination. Duplo positions the program for each arm side-by-side, using the spatial relationship between blocks from each program to represent parallelism in a way that end-users can easily understand. This design was proposed by previous work, but not implemented or evaluated in a realistic programming setting. We performed a randomized experiment with 52 participants that evaluated Duplo on a complex programming task that contained several sub-tasks. We compared Duplo with RobotStudio Online YuMi, a commercial solution, and found that Duplo allowed participants to solve the same task faster and with greater success. By analyzing the information collected during our user study, we further identified factors that explain this performance difference, as well as remaining barriers, such as debugging issues and difficulties in interacting with the robot. This work represents another step towards allowing a wider audience of non-professionals to program, which might enable the broader deployment of robotics.