This study examines corporate portfolio implementation in private equity (PE) "buy-and-build" platforms by distinguishing resource similarity and complementarity. While RBV links relatedness to performance, it conflates these dimensions, overlooking their distinct managerial demands. Integrating the attention-based view (ABV) with RBV, we explore how similarity and complementarity create attention trade-offs that complicate portfolio development. Using novel PE-specific measures—financial correlations for similarity and input-output interdependencies for complementarity—we analyze a proprietary dataset of PE platforms. Results show that platforms prioritizing either dimension implement faster than unrelated portfolios, but extreme relatedness increases implementation time, aligning with ABV predictions. Mechanism tests highlight external stimuli: cost pressures (e.g., high taxes) drive similarity-based scale expansion, while revenue opportunities (e.g., GenAI boom) propel complementarity-driven scope strategies, especially in digital sectors. This study advances corporate portfolio theory by addressing measurement challenges and highlighting trade-offs between scale and scope synergies in inorganic portfolio development.