This study explores the consequences of synchronized false data injection attacks (FDIAs) on voltage behavior in smart microgrids that rely on renewable-based distributed energy resources (DERs)—a cornerstone of modern smart city energy infrastructure. Employing both simulation-based and real-world experimental techniques, the work introduces a unified framework for analyzing and defending against coordinated threats, thereby safeguarding the reliable operation of renewable-integrated DERs, which are central to today’s urban power ecosystems. In the proposed scenario, multiple attackers independently select the most effective moments to execute two complementary cyberattacks, one producing undervoltage and the other causing overvoltage within the same smart microgrid. Each attack model is driven by an objective function that maximizes the severity of voltage deviations—either excessive undervoltage or harmful overvoltage—induced by its corresponding FDIA. Under this coordinated scheme, the adversaries construct a multi-objective optimization problem (MOOP) that simultaneously provokes significant voltage violations across the microgrid’s most exposed zones. Due to the competing objectives in this MOOP, a Pareto-front approach is applied to generate a set of non-dominated optimal solutions, along with a single best compromise solution (BCS). The potency of this combined FDIA strategy is substantiated through detailed simulation studies and laboratory-based experiments. Specifically, the well-known IEEE 13-node test feeder was reconfigured to represent a microgrid environment for the numerical simulations, while the physical validation utilized a small-scale hybrid photovoltaic/wind microgrid testbed featuring actual renewable generation sources.