Viewport viewport = new Viewport();
viewport.Width = graphics.GraphicsDevice.DisplayMode.Width / 2;
viewport.Height = graphics.GraphicsDevice.DisplayMode.Height / 2;
viewport.X = graphics.GraphicsDevice.DisplayMode.Width / 2;
viewport.Y = 0;

// set the viewport
graphics.GraphicsDevice.Viewport = viewport;

// render here

// switch to another viewport if necessary and render again