Hi Diego,
While checking your sample we have found that you have set the DockToFill as True for all the child. If we set the DockToFill as True, It will occupy the Free space in Container.
We have modified the sample as per the requirements explained in your screenshot. Please have the sample from below location.
Sample link :
DockingManager
In this sample We have set the DockToFill as True for Child named "DmControl" for occupy the free space.
Regards,
Muthukumar