Idea Summary
It would be nice to have a more comfortable way to specify the value of the attribute “Additional Outputs” of Popup LOVs. 

Use Case
Currently it is very error prone to enter multiple values in the attribute. For instance a space after a comma leads to no error when saving the page or at runtime. But the item to the right of the colon is set to null because presumably Apex cannot find the specified column because the space is not discarded. On the other hand other attributes which accept a comma-separated list (e.g “Items to Submit/Return” for Dynamic Actions of type “Execute Server-side Code”) have no problem with spaces. 

Preferred Solution (Optional)
The new dialog could look like the “Set Items” region of the Link Builder:

So on the left one selects the column of the LOV and on the right the Page Item to return the value into.

Also at least the column names in the value should be checked when saving the page.