I don't think they would be allowed to double head with 91s (or any other electric loco) anyway, since it would fry the overhead. Freightliner can only get away with it for 86s since they have to keep the amps down (the 2 locos are more for adheasion and tractive effort than raw power, hence why diesels can do the same trains single), and I don't think they can for the more powerful 87s and 90s. Also, at high speeds the second loco would have serious problems keeping it's pantograph in contact with the overhead, and I think any train double headed by electric locos is restricted to 75mph. Then again, with a second loco dead in train they are probably restricted somewhat anyway, possibly to 100mph, maybe less when running in lower speed limits.