I'm not sure whether the OP has 2 ten-hour flights and is suggesting one person sits in J for each leg, or if they have 2 ten-hour flights and they swap once per flight.

In any case, I think this is academic, since I can't think of many airlines/routes where one airline sends the same plane on one ten-hour flight and then another. BA LHR-SIN-SYD is one example.