If they get back together, then do they join to create the old photon? What if you stick one in a stronger gravitational field for a while and then later you stick them back together but maybe the phases of something are out of sync due to time dilation (or maybe they're in sync but just one has experience an extra second of life)? Does the resulting photon become entangled with the second-ago rendition of itself? Are we able to control an entangled photon well enough to run a process like this?
Sorry for the interested bystander questions.