As others already suggested, if you want to use complex functionality like sql UNION then use custom query - that's what the component is used for.
If you don't want to deal with custom queries you can use the array list options.
Create an array list, add two database queries, each of them querying your two tables. After that use two Add All In Array list steps the first one adds the data returned from the first db query and the second one adds the data returned from the second db query.
Then output the list value - you will see two results joined in a single list.
