Hi Laurent Lystig,
Define your domain module as below
For the view contacts action button, define the show page activity as below
The contacts grid should have the datasource from the association
2.To show both in a single page use the listen to widget option
Refer the below learning path as well
This can be achieved using a single page.
Create a page and add a layout grid and split the layout in to two equal halves.
In first half of layoutgrid, Add a grid and connect the Company entity.
In the second half of layoutgrid, Add a dataview and select datasource as Listen to widget. Seect the datagrid which you created in first section.
Inside the dataview, Add one more datagrid, connect the entity with the contacts enitty.Use X path as datasource,display the data using current object.