Augmented reality (AR) platforms are evolving to support immersive 3D experiences. Most modern AR platforms support only a single immersive app at a time, but users may also benefit from the ability to engage with multiple apps at once. The ability of different apps to simultaneously augment a user's world raises critical questions: how might apps visually conflict with each other, and how can we design AR platforms to support rich behaviors while mediating conflicts? In this work, we pose and explore these questions, identifying means of visual conflict as well as platform design strategies to mediate conflicts. We then analyze state-of-the-art AR platforms (HoloLens, Magic Leap One, and Meta 2) to understand their trade-offs and identify unexplored gaps in the broader design space. Our exploration reveals key guidelines and lessons to inform future multi-app AR efforts.