We use an analytical model to study the effects of customer-specific synergies – i.e. synergies that arise when firms sell multiple products to the same customers. At the firm level, we show that the profitability of a customer-specific synergy depends upon cross-market correlation of customer preferences, differs when the synergy is cost-based versus differentiation-based, and can be negative even when the synergy is kept proprietary to a single firm. We also show that returns to imitating such a synergy may decline as it strengthens. At the industry level, we find that exploiting customer-specific synergies causes endogenous market convergence at a point that depends upon whether the synergy is cost-based or differentiation-based and whether it is imitated.