We are exploring whether two currently mass marketed wearable devices, the smartwatch (SW) and head-worn displays (HWDs) can replace and go beyond the capabilities of the mobile smartphone. While smartphones have become indispensable in our everyday activities, they do not possess the same level of integration that wearable devices afford. To explore the question of whether and how smartphones can be replaced with a new form factor, we present methods for considering how best to resolve the limited input and display capabilities of wearables to achieve this vision. These devices are currently designed in isolation of on another and it is as yet unclear how multiple devices will coexist in a wearable ecosystem. We discuss how this union addresses the limitations of each device, by expanding the available interaction space, increasing availability of information and mitigating occlusion. We propose a design space for joint interactions and illustrate it with several techniques.