If you read the complete rules for back-to-back rides, you will see there are two ways to handle any extra time between the first leg and the second leg.
One way to do it is to start the second leg immediately after completing the first leg. So if you complete the first leg in, say, 44 hours, you turn around and head back the other way immediately. The receipt you obtain at the end of the first leg also acts as you start receipt for the second leg. The down side of doing this, of course, is that you could have used that extra six hours for rest with no penalty.
The other way to handle it (and the one most prefer) is to do what I just suggested - take the extra time for rest before starting the second leg. You should pick up a receipt at the end of the first leg to show you did it within the time limit, but doing so does not start the clock for the second leg. That happens when you pick up the start receipt for the second leg
(which needs to be no later than the 50-hour mark). You could continue your rest at that point, however any additional time you take after the 50-hour mark is on the clock for the second leg. But if you need the rest, you need the rest.
The complete back-to-back ride rules are at
http://www.ironbutt.com/themerides/bbseries/
Ira Agins
Iron Butt Association