All you have to do is use a mu cable between the locos (from Aristo) using the existing connectors coming out from the locos.
Put the track/battery switch in battery.
There is a plug on the board inside to connect the batteries to, if I remember correctly, or else feed an unused MU cable "end".
Now that's the way Aristo intended it. Unfortunately, they often put a resettable fuse inline with the MU "bus" inside the loco, and this fuse cannot handle the current for TWO locos. (doh!).
The resettable fuse is about 3/8" square, thin, and orange in color. You have to move it so that it only protects the loco involved, not control the current for all locos. Wiring this will vary according to the loco model, and also the phase of the Chinese moon (Aristo products have random variations in wiring, especially wire color).
You really have to check the wiring in YOUR locos to be sure. Pictures would help, but to be SAFE, you need to understand how your loco is wired, or have someone do it for you.