I can tell you from experience that you cannot effectively link two motors together with one controller. I have tried it with my RC helis. I also did alot of research into it a couple years back. Even the double AXI requires two ESCs to work.
But, the up side is, two small controllers are only slightly more expensive than one big one and you get redundancy (if one fries, the other will still run).
I have cure the belt skip issue with one wide 15mm belt. However, my efficiency went down. It did not go down much. But, a small amount. I do not have numbers for you because my logger bit the dist from overamping it. I have a 150 amp logger coming at teh end of this month.
The up side is, the whole system is running smoother and quieter than ever.
Anyway, yes there are cumulative losses with a double reduction like mine. That is undeniable. I have looked into doing a single reduction using a readily available 12 tooth motor pulley and a 72 tooth secondary pulley driving a freewheel to a left side chain to the disc brake flange on the left side of the hub. That seems to be the best solution to this. The only reason I did not go that route was space constraints under my seat. The big 72 tooth pulley would not fit.
At any rate, I am not an electronics expert. I am a self taught mechanical designer/problem solver/engineer of sorts. If it is mechanical, I can probably deisng, debug and refine it. All the technical jargon regarding motor IO, efficiency curves and the like are foreign to me. The best I can do is build it, run it, and measure AH useage over a give distance to calculate rage per charge.
Oh, I have found a problem with the AXI motors (I mention this in my build page on the web site). The AXI motors have a strange trait. They put off a strange back EMF issue from about 2800 RPM through 3200 RPM. In that range, the back EMF is sketchy and the ESC gets confused easily. So, through that RPM, I must back off the throttle so the motor doesn't "Sing" and lose power there. I just back off for a second and get back on it as it transitions through that RPM range. I spoke with Castle Creations about it and they said this is the only motor that does that. They said they could program the software around it, but they will not just for one brand motor. So, for now, this is an annoying (though admittedly minor) issue.
Other than that, I have the bike pretty well refined at this point. I am glad I built it. But, my next setup will definately use a single stage reduction to increase efficiency.
Sorry for the long post!
Matt