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But if you put a motor on each channel, and the IR receiver can't handle that from one battery box, then LEGO designed a completely piss poor product and that is not really likely in my opinion.
In the Rock Crawler set (9398) LEGO uses two PF large motors (0,49A) on one IR channel.
Where @graphite says the average draw would be less, I think he means that because a second motor is helping, the first motor doesnt need to do as much work to get the train moving. Yes, the overall current draw will be more, but as @aap points out with the specs, its still within tolerance.
I have run one #7939 with its rolling stock, and added on #7938 at the front and back and also the toy story carriages. It pulls everything with no problem. In fact the main problem is the strength of the magnets between carriages. If there is too much weight then it can detach.
I suspect that this would drain power faster though, as there is more weight being pulled round with two battery packs. Of course, it would seem like it lasted longer because that extra drain is split between both packs. If you are limited to just recharging 6/8 cells at a time then this may make a difference. Having said that, rechargeable AAAs are not too expensive so you just need to keep loads charged up.
The method @oldtodd33 uses should stop this. (But I still like having just one powerful engine. Must be the Clarkson effect)
I tried 2 configurations on a single locomotive:
1. 2 Lego motors on one locomotive with the front powered truck facing backward. I started here since it was the easier modification. The wire easily went up the hole in the middle of the engine. I attached the plug to the second port on the IR receive so I could control it separately. I had a very hard time getting the motors synced for smooth operation, since they wanted to run in opposite directions.
2. 2 Lego motors on one locomotive with the front powered truck facing forward. Power wire would have to come over the front of the baseplate and through the cab to reach the IR receiver. You could cut a hole in the base plate to let the wire through but I did not do this yet. The plug was stacked on top of the other motor plug so they would both on the same channel. This configuration showed a lot of promise, except that it would require extensive remodeling of the front end of the locomotive. Had good pulling power.
Bottom line is that it is very hard to operate 2 motors if they are not facing the same way either on one locomotive or two.
I just had a thought that I might try. To achieve the back-to-back operation that I wanted, I could put the powered truck on the front of the back locomotive. Since its facing backward it would push the locomotive in the right direction and I would not have to make major structural modifications.