What you want is not a join, but a UNION:
SELECT RecordID, UserName, NULL AS CustomerName FROM UserRecord
UNION
SELECT RecordID, NULL AS UserName, CustomerName FROM CustomerRecord
... which just appends records from the two tables.
I'd just add that the order will not be the one you have shown in your expected result. If order is important then you should SELECT from this UNION and add an explicit ORDER BY clause on this outer select. Something like:
SELECT * FROM (
SELECT RecordID, UserName, NULL AS CustomerName FROM UserRecord
UNION
SELECT RecordID, NULL AS UserName, CustomerName FROM CustomerRecord
) ORDER BY RecordID, UserName, CustomerName