I think that there is a bit of confusion when you read the specifications.
I think that it means that the only battery (a 3 cell 41W battery in the tablet), will last 6:35 hours with the dock connected and 6:48 hours when it is not.
Not that there is a 3 cell battery in the tablet and a 3 cell battery in the dock.
Here is a link to the specifications.
Also for what it is worth, here is a link which says:
"I also found myself wondering why there's no secondary battery built into the docking keyboard, as seen on the HP 13t-h200 x2; with the extra battery, it lasted 7:15, and I can imagine the Click 2 Pro could easily break the 8-hour mark if it had some reserve power in the keyboard to draw on.