Glenn Gardiner says:

The worlds most popular answer for anything, that depends. Are they DC, BlueRail, LocoFi, RailPro, or DCC controlled. If DC controlled and there is a big difference, in their speed, then there can be problems. However, it would have to be a significant difference, to be a danger. More than likely the faster one will end up either dragging the slower one (if in front), or pushing it (if in back), faster than the slower ones wheels are moving. On the same token, the slower one will cause the faster one to either be held back (if in the rear), or held up (if in the front). Again, not good, but not dangerous. They can look awkward, jerky, cause derailments, and probably some other situations. If the speed is very similar then you can probably get away with it and may not even notice a problem. Remember that if you aren’t using one of the aforementioned control systems you have no control over the individual engines speed. BlueRail, LocoFi, RailPro and DCC all give you finite control over each engines speed and more importantly, the ability to MU (Multiple-Unit Train Control) or Consisting, the engines together. They become a matched pair that function as a single unit. I am only familiar with BlueRails implementation and it is fantastic. Not only is the set-up easy but you can make tiny adjustments to each engine to fine tune their performance. Some, including BlueRail, also, control the lights. The headlights will turn on, or dim, based upon the direction of travel. Headlights, for ends of engines, will not light if they are not the lead or following engine. Good luck, Glenn