After digging deeper, I find out that :
I've understand I can add several cells in a groupbox with option, I try but each time only It works for the first column
So, I've to create the first column (first Grid) with a widget inside, I don't forget to surround my grid with new Gid above, below on the right (next column) to preseve the possibility to extend my template also outside of the groupbox. After I go on the widget in my first Grid , I open the editor with its widget An I replace the Grid of this widget by a GroupBox, I save. After that, I can continue to add other widget inside my new groupbox and access also the other Grid.
Each time a groupbox replaces a grid... It's my comprehension, thanks for some advices
Marcel