According to the E+ documentation for WindowShadingControl:

When a shading device is present it is either retracted or activated. When it is retracted it covers none of the window. When it is activated it covers the entire glazed part of the window (but not the frame). Whether the shading device is retracted or activated in a particular timestep depends on the control mechanism: see “Shading Control Type,” below. To model a case in which the shading device, when activated, covers only part of the window you will have to divide the window into two separate windows, one with the shading device and one without the shading device.

Another option would be to use the recently added SurfaceProperty:IncidentSolarMultiplier object where you can schedule (or use EMS to control) the fraction of solar that a window sees. In your case, it could perhaps be 0.5 for every hour assuming the exterior shade is fully opaque. (Note: I recommend only using it in E+ 23.2 or newer, as it received a bugfix for that version.)