Policy coupling can accelerate demand-side climate mitigation strategies, such as the shift to electric vehicles. Yet, a holistic understanding of joint policy effects at the individual and system levels remains limited. Here, we combine choice experiments and attention process tracing with system-level modelling to examine how a carbon tax and climate information intervention - individually and combined - influence people's attention and decisions, electric vehicle diffusion, and public policy support. Our findings based on data from four culturally-diverse countries (Mexico, South Africa, USA, UK; N=1,589) show that policies competed for attention but had additive effects on adoption decisions. Integrating the empirical data into an agent-based model showed that combined interventions can, however, have superadditive system-level effects depending on diffusion stage. Finally, support for tax-inclusive policy packages was higher when coupled with climate information policies. Our findings show that policy coupling can enhance impact, depending on diffusion stage, target population, and national context.