In this commentary, we discuss the concerning prevalence of cannabis use among young people who use e-cigarettes. National data in the U.S., as well as our own local data, indicate that dual use (i.e., nicotine e-cigarette use and cannabis use) is more common than e-cigarette use alone. Our commentary discusses why this dual use is a major concern for public health. We argue that continuing to examine e-cigarettes in isolation is not only impractical but also problematic, as it misses out on opportunities to understand additive and multiplicative health impacts, to share cross-knowledge, and to inform prevention and treatment. This commentary calls for more attention to dual use and concerted, equity-enhancing efforts from funders and researchers.