How to make heading2 NOT restart everytime heading1 is used?

I am working in a master document which groups together 17 subdocuments, whose headings are of different levels. My main problem comes from the fact that I want the heading level 2 to NOT restart the numbering after each time level 1 is used. How can I do that? Thanks for your help

Ex :
Chapter 1

PART I contains
Chapter 2
Chapter 3

PART II contains
Chapter 4
Chapter 5

I managed to find a solution:
in each subdocument that follows heading1 in the master document (in the case of my example, chap2 and chap4), place the mouse cursor in the title of the document (style: heading2), then: Format > Paragraph > Numbering, then check the box “Restart numbering with this paragraph”, check “Start with number” and choose the correct number.
There is no need to do this in every subdocument, only the ones that follow heading1.