Hi wiggy,
according to Boeing you should try for reverse on both, regardless of the failure,....and like others - no idea why.
We have one simple RTO procedure for all runway conditions, so when the chips are down - there is a better chance that you will do it correctly.
https://www.faa.gov/other_visit/avia...off_safety.pdf
Page 2.8.
"Reverse thrust is not used to determine the FAR accelerate-stop distance (Figure 9), except for the wet runway case for airplanes certified under FAR Amendment 25-92. FAR criteria provide accountability for wind, runway slope, clearway and stopway.
FAA approved takeoff data are based on the performance demonstrated on a smooth, dry runway. Recent models certified according to FARAmendment25 92 also have approved data based on wet, and wet skid-resistant runways.
Separate advisory data for wet, if required, or contaminated runway conditions are published in the manufacturer’s operational documents.
These documents are used by many operators to derive wet or contaminated runway takeoff adjustments."
2.32.
"Most ofthe takeoffs planned in the world do not include reverse thrust credit. This is because the rejected takeoff certification testing under FAA rules does not include the use of reverse thrust, except for the wet runway case for airplanes certified under FAR Amendment 25- 92. An additional stopping margin is produced by using maximum reverse thrust. We stress the word “maximum” in relation to the use of reverse thrust because of another commonly held misconception. Some pilots are of the opinion that idle reverse is “equally or even more” effective than full or maximum reverse thrust for today’s high bypass ratio engines.
This is simply not true. The more EPR or N1 that is applied in reverse, the more stopping force the reverse thrust generates."