From elementary physics, F = ma (Force = Mass X Acceleration) shows that if F is doubled, a (acceleration) will be doubled only if the mass remains unchanged - as would be the case where the tractive effort of the single locomotive could be doubled with no change in the mass of the entire train. There is, however, no such magic on my side of the event horizon. While the addition of a second similar locomotive doubles the tractive effort (F) available to accelerate the train, the total mass of the train is also increased by some not insignificant amount. This means the acceleration will be increased by a factor of something less 2. As an example:
Assume a consist of a single P-42 locomotive and ten passenger cars each weighing 270,000 and 130,000 pounds, respectively. The total weight of this train would be 270,000 + (10 X 130,000) = 1,570,000 pounds or 1,570,000 ÷ 32.2 = 48,800 pounds mass. The tractive effort of this locomotive is between 63,000lbf starting and 38,000lbf continous so just for giggles lets say its average tractive effort is about 50,500lbf. Its acceleration will thus be an average of: a = F ÷ m = 50,000 ÷ 48,800 = 1.03 ft/sec².
Adding a second P-42 increases the total weight of the train by 270,000 lbs to 1,570,000 + 270,000 = 1,840,000 lbs and its mass to 1,840,000 ÷ 32.2 = 57,100 lbs mass. Assuming the addition of the second P-42 will double the tractive effort to 2 X 50,500 = 101,000 lbf, the acceleration of this train will be an average of: a = F ÷ m = 101,000 ÷ 57,100 = 1.77 ft/sec².
Newtonian Mechanics thus leads me to believe the addition of a second P-42 will increase the acceleration of the entire consist by a factor of 1.77 ÷ 1.03 = 1.72 and not by a factor of 2.