A main support frame is formed from sections of a plastic extrusion and has opposite side portions with peripheral recesses receiving an inner sash unit and outer sash unit each having a frame formed from sections of a plastic extrusion and supporting an insulated glass unit. Hinges support the dual sash units for pivotal movement between open and closed positions, and gear connected telescopic link members connect the main frame to the sash frames for simultaneous movement. A lock system includes a handle on the inner sash unit for moving straps with studs on the sash frames through a connector mechanism mounted on the main frame for simultaneously locking and releasing both sash units and for releasing only the inner sash unit. A screen and/or mini-blind may be supported between the sash units, and the window system with dual sash units may be constructed in various forms.