I would like to have a simple split like this in Scribus. Is there any way to do that? Note that the real shape I need to cut is more complicate than that.
For the benefit of others, and possibly in addition to the answers provided in the linked Scribus forum post, Scribus has a set of boolean tools you can use on paths.
The Scribus forum answer suggests using
Item -> Path Tools -> Cut Polygon having first selected a polygon and a bezier curve to cut it. (It doesn't work for me if the intended cut line is merely a "line")
If you're more minded to deal with combinations of volumes then select two polygon items and then in the menu go
Item -> Path Tools -> Path Operations...
I think the dialogue is pretty self explanatory:
As the operations are so deep in the menu structure and not available in a right-click menu I cursed Scribus for a while for its apparent lack of functionality until I discovered these.