The physiological response to thermal fluctuations has been a subject of investigation for decades. These studies have been fundamental in unraveling the underlying mechanisms that allow organisms to cope with temperature variations. However, when attempting to generalize organism responses to environmental changes in natural settings, we confront the inconvenient truth: the natural environment is inherently complex, characterized by diverse and variable stressors. To address this complexity, a transition from a single stressor approach to a multiple stressor scenario becomes essential. We must explore how different thermal fluctuations interact with co-variability in other environmental factors, shaping the phenotypic responses of wild organisms. While recent research has focused on predictable co-variation between stressors (e.g., heat and hypoxia), the effects of simultaneous exposure to emerging and human-driven environmental changes—such as heatwaves and toxicant releases—remain unclear. In this context, I present two case studies examining the phenotypic plasticity of thermal tolerance in adult and offspring generations of two fish species. Specifically, we investigate their responses to environmentally relevant concentrations of copper, a widely used and released metal, both independently and in conjunction with a heatwave event. Contrary to our initial hypothesis, the data revealed unexpected results: thermal plasticity was not negatively affected by copper exposure, to some extent. Instead, we observed a potential cross-tolerance effect. This suggests that exposure to one stressor may enhance an organism’s ability to cope with a second stressor. These findings offer new perspectives and inform future research directions.