Creating multiple prototypes facilitates comparative reasoning, grounds team discussion, and enables situated exploration. However, current interface design tools focus on creating single artifacts. This paper introduces the Juxtapose code editor and runtime environment for designing multiple alternatives of both application logic and interface parameters. For rapidly comparing code alternatives, Juxtapose introduces selectively parallel source editing and execution. To explore parameter variations, Juxtapose automatically creates control interfaces for "tuning" application variables at runtime. This paper describes techniques to support design exploration for desktop, mobile, and physical interfaces, and situates this work in a larger design space of tools for explorative programming. A summative study of Juxtapose with 18 participants demonstrated that parallel editing and execution are accessible to interaction designers and that designers can leverage these techniques to survey more options, faster.