Speaking from the 787 side as a C Captain you would never sign as it is a new fleet and Everyone is new on the airplane therefore the other Captain will almost definitely be an instructor. You will be the second Captain for the first year. I know on the 744 there are usually 2 foreign Captains on those flight and I am not sure how they figure out the PIC on record but I suspect they just alternate. Everyone I have talked to here in China that operate on a heavy crew split the PIC 50/50. On a 13 hour flight 1 PIC cannot log 13 hours of PIC time.