thank you for your answers! I dont wont to do a async postback each time, especially because the lists shouldnt be limited during the changes. I solved it in that way: The user clicks on save, I create a bool array for each dropdown item, then I loop through all dropdownlists in the placeholder. If there is a area from 3 - 5 I set the values with index 3-5 in my bool array to true. If one of these values is already true, there must be a wrong selection in the start/stop positions. I think, its not a very nice way but works pretty fast and does everything I want. If someone has a better idea, please let me know... ;)
Thanks! Stefan