This study explores the co‐development of two related but separate reading skills, reading fluency and reading comprehension, across Grades 1–4. A bivariate biometric dual change score model was applied to longitudinal data collected from 1,784 twin pairs between the ages of 6 and 10 years. Grade 1 skills were influenced by highly overlapping genetic and environmental factors. Growth in both skills was influenced by highly overlapping shared environmental factors. Cross‐lagged parameters indicated bidirectional effects, with stronger effects from fluency to comprehension change than from comprehension to fluency change.