Background: In dual-task (DT) conditions, individuals must walk while simultaneously engaging in cognitive or motor tasks, which impacts gait performance, especially in older adults and individuals with Parkinson's disease (PD). Gait impairments in PD under DT conditions have implications for intersegmental coordination. Research question: Intersegmental coordination and gait biomechanics during the DTs were compared between people with PD and older adults. Methods: Thirty-two individuals (16 PD, H&Y 1-3; and 16 older adults) participated in this study and were asked to walk under the following self-selected conditions: single task, DT with a math component, and texting on a cell phone. Spatiotemporal, angular, and intersegmental coordination data were collected using a markerless motion analysis system (OpenCap). Results: Dual-task conditions significantly affected spatiotemporal and kinematic variables, as well as intersegmental coordination. A significant task effect was observed for thigh-shank coordination, whereas no significant group effect was found for the main coordination outcomes. Significance: Significant task effects were observed for intersegmental coordination (thigh-shank CRP), with no significant group differences. The concurrent demands of processing visual and motor information for texting and walking lead to significant reductions in gait speed and lower limb movement, as well as altered intersegmental coordination, with task demands rather than disease status being the primary driver of coordination changes.