Here is what I wish to have:
Chapter 2 - Foo
This is an introductory paragraph, spanning over the
whole width of the document.
This is the body of the | due to the use of the
chapter which is written | 'book' class with the
on two separate columns | 'twocolumn' option.
How can I achieve this layout?
Edit: I do not want to use multicol since it would be used on each chapter to explicitely specify what should be the default behaviour of my document (i.e. two columns). As such, the referenced question is not a duplicate.

scrbookyou can do it using\setchapterpreamble. – Schweinebacke Nov 29 '17 at 13:59