title: Enforce unique values across two tables
body: <p>Is it possible to enforce uniqueness across two tables in MySQL?</p>
  
  <p>I have two tables, both describing users. The users in these tables were for two different systems previously, however now we're merging our authentication systems and I need to make sure that there are unique usernames across these two tables. (it's too much work to put them all into one table right now).</p>