I think you should handle it this way:
Retrieve both entity lists using database retrieves. Create a new empty list with Create List, then loop through the first list and use Change List (Add) to add values to it. Do the same for the second list.
Hello Nur,
To reach that you entities needs to be the same or inherit of the same
(look for on Generalization here https://docs.mendix.com/refguide/generalization-and-association/)
So i am afraid in your current construction you would not achieve your goal.
I think you should reflect on your data model, maybe write down your use case and see which entities are overlapping or are actually the same.
Feel free the ask more questions the community is here to help!
Hope this helps,
Good luck!