In a family type "Generic Model" named "child", I created references planes and a new extrusion.
The parameters of the length have to be "shared parameters" and the type "instance".
I opened a second "generic model" called, this time, "mother" and I created a new extrusion.
After, I inserted the file "child" in the mother file and I created a composition with this 2 elements.
Finally, in a new "project" I assembled them to create a bigger composition more complex.