Simple and complex contagions are typically distinguished based on whether their transmission mechanism is linear or nonlinear. Is the probability of transmission the same whether one has a single infected neighbour for 2 days or 2 infected neighbours for a single day? The latter scenario tends to be more contagious in social situations because of effects such as social reinforcement. These nonlinear effects lead to contagions with radically different phenomenology: discontinuous phase transitions, hysteresis loops, and speed up due to social clustering. However, similar phenomenology can be shown to occur in the case of simple contagions with synergistic interactions. We use this parallel to show how interacting simple contagions spreading on a clustered network look like a complex contagion if one is unaware of the interaction or not tracking the co-infections. In a social context, our results highlight the difficulties of identifying and quantifying mechanisms such as social reinforcement when innumerable amount of ideas, memes and behaviours interact on complex social networks. In the biological context, this allows us to use the toolbox of complex contagions to detect novel biological interactions between pathogens.