I am not sure that I understand your requirement. It seems that you describe the default behavior when the SelectionMode is set to Multiple - click with the left button of the mouse will select the clicked item and another click will deselect it. You can take a look at the Selection
demo and change its SelectionMode to Multiple to see this behavior. Note that the example is in Silverlight but it shares its code and behavior with the WPF version, which you can download
from our website if you prefer.
If this is not what you are looking for I would ask you to send me a drawing of the actual and the expected result. This will help me in the better understanding your scenario and assist you further with it.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.