Hi Gulden Beken
check below link for your reference-
Mendix Forum - Idea DetailsMendix Forum - Idea Details
Hope it helps
Indeed, it's not possible to add objects of different entities in one lists, except for specializations.
You can have a list of Apples and a list of Bananas, but you may not mix them. You could however make both entities specialization of the Fruit (if that makes sense), and than add them to a list of Fruits.
The most important question here: why do you want to union two lists of different entities?