It is not an easy thing to do as yogibarnes says. You will need to ensure that the locos in use have similar, if not identical, speed curves, as if not, even a count-down as you slow to a stop will be difficult. You can adjust your throttles simultaneously but if not set the same, the locos will respond differently. Even starting off can be difficult, as if the banking loco is slow beginning to move, and the train is on a curve, you run the risk of the wagons on the train straight-lining the curve. Likewise if you notice the couplers bunching, and slow down a bit too far.
Despite a number of attempts, I have not yet successfully completed this task, nor seen any other pair of operators do so, on the layouts I have operated on.