Based on my understanding what you are expecting is: rendering list of objects in to columns.
When select entity or object in list B, those are entities: like tuples.
By default, Mendix supports displaying of records or tuples or objects in a list. It is not possible to show the selected records to columnized view. Because, we must know the number of columns ahead during the development.
Lets say if you select 100 objects from list B, then you need 100 columns, which is also a bad practice to program like this.
If it is a fixed number of objects selected from list B (like 5 or 10) then we think of alternate ways to do this.
May be you need rethink your approach.
Check this : https://forum.mendix.com/link/questions/102263
It is similar question, but he is working with limited set of data.