This chapter cross-correlates investments by the AIIB and the Asian Development Bank (ADB) in ASEAN countries to determine whether the patterns of investment by these two institutions are similar, substitutive, complementary, or conditionally coexistent over time. The chapter evaluates the time coincidence and lag pattern of investment flows and project approvals by examining the relationship between an increase in the activity of one bank within the country setting and the alignment or negation of the other bank within the same context. This analysis is set against the background of institutional requirements, co-financing, sectoral concentration, and changing regional conditions, so that the chapter may identify synchronized expansion and offsetting dynamics. The results provide a quantitative measure of the relationship between the investment behaviors of the two banks across time horizons, which serves as the empirical basis for explaining the interaction between institutions in ASEAN’s development finance.