Hi Renger,
To achieve what you need, in the Setup for the Advanced section of the Filter for the view you're in, add a rule where (DueDateTime on or before Today +14). Then add another rule underneath where DueDateTime does not exist
The final edit to make is to click on the first rule and make sure the final option at the end of the rule is set to OR - that will mean that any tasks in the view meet either of the rule criteria, not both.
Hope that helps
Richard