It appears from the image that you use a "negative" plane methodology - black in the image is metal and white indicates an absence of metal.
It is easier to implement star grounds using this method that it would be for "positive" planes.
Use the line draw tool instead of the rectangle tool to fence in the one ground (i.e. AGND) but to leave a gap at the point where you would like the two grounds to connect.
Please refer to the attached image.