Why do you think the battery lasts twice as long?
In a properly functioning loco, there is virtually no difference.
two motors, each will do half the work a single motor will do... it's a basic law of physics.
The fact that all your motors work at different speeds means they are damaged somehow, or you have bearing problems. I would invest the time to fix them right, as opposed to running them until they fail and you have to replace EVERYTHING.
I'm going to keep reminding you that you say you don't like learning things the hard way, well your current course is the hard way again, honestly.
Since it's winter, a perfect time to learn, rebuild the motor blocks, find out what is wrong and get at least one loco running right.
Greg