Update: see my C# GDI+ code that does what you want. You can easily write the same in C++: the GraphicsPath class is merely a thin wrapper over the corresponding gdiplus.dll functions.

static class GraphicsPathExt
{
    [DllImport( @"gdiplus.dll" )]
    static extern int GdipWindingModeOutline( HandleRef path, IntPtr matrix, float flatness );

    static HandleRef getPathHandle( GraphicsPath p )
    {
        return new HandleRef( p, (IntPtr)p.GetType().GetField( "nativePath", BindingFlags.NonPublic | BindingFlags.Instance ).GetValue( p ) );
    }

    public static void FlattenPath( this GraphicsPath p )
    {
        HandleRef h = getPathHandle( p );
        int status = GdipWindingModeOutline( h, IntPtr.Zero, 0.25F );
        // TODO: see http://msdn.microsoft.com/en-us/library/ms534175(VS.85).aspx and throw a correct exception.
        if( 0 != status )
            throw new ApplicationException( "GDI+ error " + status.ToString() );
    }
}

class Program
{

    static void Main( string[] args )
    {
        PointF[] fig1 = 
        {
            new PointF(-50, 0),
            new PointF(0, 50),
            new PointF(50, 0),
        };

        PointF[] fig2 = 
        {
            new PointF(-50, 25),
            new PointF(50, 25),
            new PointF(0, -25),
        };

        GraphicsPath path1 = new GraphicsPath();
        path1.AddLines( fig1 );
        path1.CloseAllFigures();

        GraphicsPath path2 = new GraphicsPath();
        path2.AddLines( fig2 );
        path2.CloseAllFigures();

        GraphicsPath combined = new GraphicsPath();
        combined.AddPath( path1, true );
        combined.AddPath( path2, true );
        combined.FlattenPath();

        foreach (var p in combined.PathPoints)
        {
            Console.WriteLine( "<{0}, {1}>", p.X, p.Y );
        }
    }
}