I found these two motionless on the side of a small bridge by a creek in the woods. They had been in this position for at least 2 1/2 hours. Not sure how long they were like this before I found them, but hadn't moved at all since I saw them in the morning and returned a couple hours later with my camera. Once they parted, the smaller fly quickly flew off, and the other larger (female?) remained motionless for another half hour or so. Got some shot of possible fertilized egg that was on the backside of the larger fly. When ready, it started to move slightly and then walked along the railing. Not sure if it ever flew away.

Note that the smaller fly that is hanging upside down is also face up.