Double condition task

I want to add a supervisory task (Task 3) with no specific duration other then supervizing Task 1 and Task 2. Let’s say Task1 starts on Jan 1st until jan 15th and Task 2 starts on jan 10th until feb 1st.
I create Task 3 with 2 dependancies : Task 1 as a Début-Début and Task 2 as Fin-Fin (this should force the start date and the end date of task 3).
Then Gantt diagram displays the relations but Task3 is not having the duration from jan 1st to feb 1st (should be 32 days). Start date and end date are actually both on feb 1st.

Am I misunderstanding something ?

GanttProject does not change the durations of regular tasks, sorry.

Ah, ok, so there is no possibility to use “elastic task” that would adapt to unavoidable changes in the life of a project ? Like supervision or follow-up ?

Yes… Simulating with GP in “summary tasks” as shown in those posts.

Ok, I see. Using 2 “half-tasks” do the job.