check that new username is in neither of two separate tables
SELECT * FROM xymply_users, xymply_newusers
WHERE xymply_users.user ='test'
OR xymply_newusers.user ='test'

(SELECT 1 FROM xymply_users WHERE xymply_users.user ='test')
UNION
(SELECT 1 FROM  xymply_newusers WHERE xymply_newusers.user ='test')

SELECT * FROM xymply_users.u, xymply_newusers.nu where u.username = nu.username

SELECT * FROM xymply_users.u, xymply_newusers.nu where u.username = 'username' AND nu.username = 'username' AND u.username = nu.username