Last tested: Jan 21, 2019
You can only supply one field to this parameter. To get values from more than one field create a derived table, union all the columns you want suggested, and then use this new dimension as the suggester. Remember to select a key in this as well, because you must join this view in the explore in order for it to work.
# SQL for the derived table
SELECT id, field_1 AS suggestions FROM table1
SELECT id, field_2 AS suggestions FROM table1
# suggest dimension must be fully scoped