Traditional models of slow slip events (SSEs) oversimplify fault geometry, although imaging shows subduction faults are segmented and complex. We examine how fault interactions control slip behavior using 3-D quasi-dynamic earthquake sequence simulations of two parallel faults with uniform rate-weakening friction accelerated by hierarchical matrices. Four regimes emerge-periodic earthquakes, coexisting SSEs and earthquakes, only SSEs, and complex sequences -whereas a single planar fault produces only earthquakes. We quantify interaction using the maximum Coulomb stress induced on a target fault by a spatially uniform unit stress drop on a neighboring fault. Because the stress drop is normalized, the metric depends only on geometry and is independent of friction or nucleation length, allowing extension to arbitrary fault systems. SSEs occur only within an intermediate interaction range. Simulations reproduce observed moment{duration scaling and show sensitivity to detection thresholds. These results demonstrate geometric complexity alone generates both slow and fast earthquakes through evolving traction heterogeneity.