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.