It's a bug, in the property definition file Signal is set as a single entry you need to change this to multiple. Open the Central Library from the toolbar and in there open the Property Definition Editor. Find Signal and expand the dialog by pressing the Advanced button and modify the Instances Allowed from Single to Multiple.
Close the Library tools and run Tools - Update Libraries to refresh the property list. You should then be able to modify both strings without them disappearing.
If the Tools - Update Libraries doesn't work just close and re-open xDX Designer.