Projector phones, handheld game consoles and many other mobile devices increasingly include more than one display, and therefore present a new breed of mobile Multi-Display Environments (MDEs) to users. Existing studies illustrate the effects of visual separation between displays in MDEs and suggest interaction techniques that mitigate these effects. Currently, mobile devices with heterogeneous displays such as projector phones are often designed without reference to visual separation issues; therefore it is critical to establish whether concerns and opportunities raised in the existing MDE literature apply to the emerging category of Mobile MDEs (MMDEs). This paper investigates the effects of visual separation in the context of MMDEs and contrasts these with fixed MDE results, and explores design factors for Mobile MDEs. Our study uses a novel eye-tracking methodology for measuring switches in visual context between displays and identifies that MMDEs offer increased design flexibility over traditional MDEs in terms of visual separation. We discuss these results and identify several design implications.