Hi, did you ever find an answer to this problem? I have just run into the same problem.
I have a list of charts which a user selects to view the chart which are all in the same space. If nothing is selected in the list box then the space is blank and I have got a message saying 'please select which chart you would like to see.' If one option is selected the appropriate chart opens, if two options are selected both open but are overlaid each other.
I'd like to stop the user selecting two items, which is easily done with the 'Always One Selected Value' field but I would also like them to not have any selected.