What you need here is "Union", which is coming in version 9.3. (Should be released very soon - it is wrapping up beta now)
In the meantime, an alternative might be making a third datasource with a list of all of the unique values for "Invoice Site".
Then, use that as the primary and join the other two datasources to it. You can even combine them into one field with IFNULL.
(Or, even better - combine the two sheets outside of Tableau)
Hope this helps!
Thanks Derrick, I'll give the 3rd datasource option a try. That "union" option sounds intertesting, can't wait to see that in 9.3