Kiwi - I think our riding experiences with dual motor rigs are very similar. However, I chose to use the same BMC V2S gear motors front and rear so the load would always be equally shared regardless of load, speed, and throttle setting. This may not be necessary, but it has worked out well and the result is a system that reaches a few percent higher speed on the flat above what a single motor will do, but gets there PDQ. The nifty part is the available torque for hill climbing: Running rear-only, a hill may cause the bike to grind down under 10mph, but kicking in the second (front) motor will suddenly accelerate up the hill. Having the manual thumb toggles mentioned above lets you switch to two motors in advance so you don't actually ever experience this kind of slowdown and recovery... when you eyeball an impending demanding climb, you just back off the throttle briefly, kick in the second motor, and sweep up the throttle to maintain the current speed smoothly but with twice the torque on tap. With freewheeling gear motors, it all happens in a blink without a glitch.
I run the motors current limited to 800W each - 33% above the rated 600W but they are never more than warm to the touch. This dual motor approach seems a good way to easily and safely pull 1600W while keeping the ability to be thrifty on the flat. By suppressing the CA limiting for a few minutes, you can easily pull 2.4KW to crest tough hills. I have a solid 30+ mile range on a 48v 20Ah Headway pack (to 80% discharge) over mixed hilly/flat New England terrain with occasional short 15+% grades. Time will tell, but the rig does not heat or protest in any way and operates smoothly and effortlessly.
There is really not a lot of available descriptive material on dual motor builds but it seems safe to say that many of the conventional anecdotal experiences with single front motor builds may not be directly applicable - a dual motor build is (or can be) a different animal in the same manner that AWD cars offer advantages over certain (similar) troublesome control and traction characteristics of FWD.
For those readers that are leery of reported gear motor failures (stripped gears or melted windings), I can only note that most appear to have arisen from gross overpowering and high power dead starts. A Cycle Analyst is a 'must' and can easily limit power to keep these torquey motors in the specified operating envelope, making them an attractive choice for hauling a heavy cargo bike up substantial hills. I use a single standalone CA that limits total power to 800W when running a single motor and 1600W when switched to two (on the fly) with a common battery, shunt, and CA (this gimmick would not be as effective if the front and rear motors/controllers/wheels were not identical...).
(and then share it here for others to improve on)
. Fortunately the boxes are intentionally oversize for battery experiments and the upper impaled 1/3 was empty of vital organs...
