If you have directional lighting in both A units, the back one may always be faster than the front unit. When the lights are not turned on, extra power is available for the motor causing the reverse facing unit to run faster. If that is the problem, you may need to install a resistor in series with the motor on the rear unit to simulate the power draw of the light. The other possibility would be to use dummy couplers glued together to make a draw bar between the units. There is another possibility. I model in ho scale so am not familier with n scale locomotives, but usually ho scale a units use the same frame as a dummy b unit. If that is the case in n scale, you might be able to put the b unit on the second a unit chassis, removing the headlight if necessary, and then put the second a unit shell on the dummy b unit chassis and use it as a trailer behind the powered a-b set.