C++ namespaces and defining classes in separate files

Yes, you can split the namespace into multiple blocks (and hence files). Your classes will belong to the same namespace as long as they are declared in the namespace block with the same name.

// Canvas.h
namespace TwoEngine
{
    class Canvas
    {
        // Define all of Canvas here
    };
}

// Primitive.h
namespace TwoEngine
{
    class Primitive
    {
        // Define all of Primitive here
    };
}