the body layout part can not have a scroll bar of its own.
The window has a scroll bar
As a work around, you might use form view with two portals. That will require a relationship that works for your top list of records in order to get them into a portal, that may or may not be easy to do.
This may not fit your need looking at your layout screen shot, but you can have a large header or navigation part in a list layout. Although the scroll bar will scroll the full height of the window, only the list below will actually scroll, providing a fixed area at the top.
In the attached screen shot the black area (Top Navigation, Title Header or Header part) will remain static, whilst the list below will scroll. However, it does not provide 2 independent lists.
Use Two portals. One portal (top or side) could be the master portal. Click on a line in that portal and the other portal now shows related records.
The first portal line would be a button that sets a $$variable to a value in the record. The second portal would have a filter to select only records matching $$variable.