Well, you don't necessarily "target" them. Because the returned recordset will be both of the sets combined, it will loop through each row for the transformation. Because of that, you can just specify "DocumentName" and it will apply to both. Because each type of page has that field in it, you'll get a value just by using the field name.
If you are referring to specifying the field in the WHERE condition, it doesn't work because of the implicit and explicit joins that are happening behind the scenes in Version 9.
It is worth noting that in Kentico 9, the page-specific views were removed.
If you need to do a column-specifc WEHRE, it will not work because Kentico will apply the WHERE condition to all of the page types behind the scenes.
You could, however, target them in your transformation. You can select "ClassName" as one of the Selected Columns and do a if Eval("ClassName") == "CMS.Blog" type of statement to conditionally show / hide some values or something.