I discovered the footage in the documentary film, Megafactories: Boeing 747 (National Geographic):
http://www.newairplane.com/747/testflight/#/4
The 2 unknown objects (flying side by side) can be seen (in the upper left corner of the screen, between the clouds) at the moment the narrator says (halfway into the video): “In this clip from August, the pilots purposely put the freighter into a stall.”
I don’t know if the footage is recorded (unknown date) over California, Washington State or somewhere else.
Boeing 747-8F
(wikimedia.org photo)
