I've created a control designer according to this blog post http://www.sitefinity.com/blogs/josh-morales-blog/2011/09/20/creating_advanced_sitefinity_4_widget_control_designers
It works fine until I added 2 RadDatePicker controls. When there are 2 datepicker controls on the widget designer and their ClientIDMode is set to Static they don't work. If you click either one, the date is set only in the last datepicker. If you set their ClientIDMode to AutoID all works fine.
I'm using version 5.3.3900.0