Referring to this post in my comment above. Thanks for any replies.
Mine is still in pieces (yet another of half-started projects), so I haven to run mine beyond the initial test drive.
Each complete engine is self-contained 0-8-0 locomotive, but electrically they are interconnected, and also connected electrically to the tender.
If you want to do some troubleshooting, place the locomotive upside down, take couple of test clips connected to the cranked up throttle (assuming you have a DC operated model) and touch them to just the pair of front engine's wheels, just the rear engine's wheels, and just to the tender wheels. I suspect that the front engine will be running when the test leads touch the front engine's wheels, rear engine's wheels, and tender wheels. Is that what you observe?
If that is the case then the problem is with either the rear engine's contacts which connect the rear motor to the rear engine's split frame, or some sort of mechanical bind in the rear engine.