Within the prevailing single-kite paradigm, the current roadmap towards utility-scale airborne wind energy (AWE) involves building ever larger aircraft. Consequently, utility-scale AWE systems increasingly suffer from similar upscaling drawbacks as conventional wind turbines. In this paper, an alternative upscaling strategy based on stacked multi-kite systems is proposed. Although multi-kite systems are well-known in the literature, the consideration of stacked configurations extends the design space even further and could allow for significantly smaller aircraft, and therefore possibly to cheaper, mass-producible utility-scale AWE systems. To assess the potential of the stacking concept, optimal control is applied to optimize both system design and flight trajectories for a range of configurations, at two different industry-relevant wind sites. The results show that the modular stacking concept effectively decouples aircraft wing sizing considerations from the total power output demand. An efficiency increase of up to 20% is reported when the harvesting area for the same amount of aircraft is doubled using a stacked configuration. Moreover, it is shown that stacked configurations can more than halve the peak power overshoot within one power cycle with respect to conventional single-kite systems.