Otherwise you can just chain them, and travel further than 6 hours, or more than 6hours over a day.
I expect that you can chain them, but you still can't travel (or even send information) more than 6 hours back in time. That restriction is part of the nature of time, not a limitation of time turners, and a chain of time turners can't break it any more than an individual time turner can, even if you were to turn it 7 times.
That's my interpretation, anyway.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.