What would be the best approach to solve this problem? Bob.
But can you define the problem? If you get 1% more battery running time is that worth $350 of electronics...less the value of a beer? How about 2% more battery time? Etc.
From reading posts on OGR for years there's not much discussion about power efficiency and need-to-know about where the Watts are actually going. With growing interest in battery power I suppose this will change. Here's my take. The mechanical output Horsepower vs. electrical input Watts efficiency of the motors used in these diesels runs about 60-70% at peak efficiency. For example, here's a Mabuchi RS-385 which one might see in an O-gauge engine. With 12V applied, peak efficiency is about 65% delivering 80 g-cm of torque spinning at 8200 RPM. Do the math on your engine (gear transmission ratio, wheel diameter) to get scale-MPH. Note how the efficiency drops off dramatically when you go faster or slower than the sweet-spot zone.
Picking off more numbers this is 9 Watts of electrical power in, delivering about 7 Watts of motive power into the transmission. So that's 18 Watts for two motors...and we know that you can easily be drawing an incremental 18 Watts of power off the transformer when the diesel starts pulling. In other words for a typical O-gauge diesel pulling, say, 3 Amps (54 Watts at 18V AC command voltage), this motor would NOT be operating near optimal efficiency. One would have to run some experiments but I'd guess that power wasted due to mis-matched motors is less than 10%; if someone can provide a better guess I'd like to hear it!
In other words you're looking to save a few Watts. You could save that much power just turning off smoke. Or you could lower the volume and save a Watt or two. And like improving MPG in your vehicle, acceleration/braking habits can improve efficiency more than 10%. Are you using a mainline diesel for switcher duty? The gearing is different to allow the motors to operate closer in the efficiency sweet-spot which I'd think could double running time rather than just get you 10% (or whatever). I have no idea if manufacturers select motors to maximize efficiency...but you could. If you have money and time on your hands perhaps investigate swapping motors. It may require different electronics to drive, say, brush-less vs. can DC motors. Or if you are the mechanical type you can swap in a different gear-box to operate at maximum motor efficiency for the speed that you most operate your engines.
So even if you had separate tachs and separate motor drive electronics (which itself consumes power that must be accounted for) I'm not sure you would see improvement in battery run-time.
Is this PS3 on battery? If that's the case and you're willing to muck around with the electronics, I'd think you could save several Watts of power by bypassing the AC-to-DC bridge-rectifier that's on PS3 boards. Since PS3 boards can run on DC, it seems you could save the lost power in the diode drops. So 3 Amps dropped "unnecessarily" across two 0.7V diodes is 4 Watts. I'd think that is more power saved than a motor balancing scheme.
All this just my opinion of course...