How to use multiple LEFT JOINs in SQL?

Yes it is possible. You need one ON for each join table.

LEFT JOIN ab
  ON ab.sht = cd.sht
LEFT JOIN aa
  ON aa.sht = cd.sht

Incidentally my personal formatting preference for complex SQL is described in http://bentilly.blogspot.com/2011/02/sql-formatting-style.html. If you’re going to be writing a lot of this, it likely will help.