Tonight I did a little research and believe this is the answer to this Jennings-Hess mystery on WTC7.
The time when WTC7 lost power is also the time Mike Hess claims that he and Barry Jennings left the OEM on the 23rd floor and started down, where on the Eighth Floor they experienced that explosion. He says the time they started down was when the building lost power when they were on the 23rd Floor.
This can be heard in this short, 30 second video clip:
http://www.youtube.com/watch?v=FQ8PzzPLRJo
However, in FEMA's report on WTC7, on page 5-15, Con Ed reported WTC7 lost power at 9:59 (due to the collapse of WTC2):
http://www.fema.gov/pdf/library/fema403_ch5.pdf
Jennings apparently got flustered by the events, understandable under the circumstances.
He and Hess experienced the explosion when they got down to the 8th Floor (going down the stairwell due to the power being out), and this occurred after WTC2 had already fallen...and this explosion they experienced must have been very close to the time when WTC1 came down (going down from the 23rd to the 8th Floor would take some time).
Regardless, it happened after WTC2 fell.
This means Jennings could not possibly have seen
both towers standing when he looked out after the "explosion" (especially through all the dust and smoke from the collapse).
He also showed his being flustered by the events when he stated this happened when they got down to the 6th Floor, but in the referenced interview above both he and Hess said it happened on the 8th Floor.
Little things can slip in the mind during a harrowing experience, and I believe this matter is closed to my satisfaction.