This is incorrect. 2wd does not provide a speed increase (with a minor exception noted below). Just for starters, geared hub motors have rpm limits and those don't change. So the motor spins as fast as it is going to spin regardless of how many of them you hook up in a line together.
With that said, a 2wd dual-Bafang G060 bike with twin 35a controllers - one for each motor - hooked up to a single battery (52v, 30ah with a 90a continuous BMS) on 26" fat tires will peak at just barely 34 mph. Using just one motor on the same bike, its peak will be 32 mph. The 2wd will take a bit to get up to 34 from 32 as well.
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On pavement, what WILL increase is your level of acceleration to any speed from zero to the peak. In the left turn lane on my hub+hub commuter, I need the granular control of dual throttles to keep myself from running into the bumper of the car in the turn lane in front of me. Its safest in that one scenario to stay away from dual PAS and use throttle initially to be sure you control your start. Rear throttle only for a blink, then hit the front. 2 seconds in your pedaling takes over and you lift off both throttles and complete the turn without running into anything.
There is much more to it than that.
Two motors working together... one alone will get too hot to touch. Two together and they are barely warm. The linked video in the opening explainer above gives the specific reasons why this is not an illusion and there really is a large synergistic effect in terms of heat reduction.
Here's one more 2wd benefit: My 'sand crawler' was a flat-ground commuter for about its first 1300 miles. That was my first mid+hub 2wd, built because I learned a hub+hub (the orange one above) was not survivable long term in steep hill country. I discovered the front motor completely wipes out any increased drivetrain wear that comes from a high powered mid drive in the back: The mid is never hauling the bike up from a dead stop anymore. On flat ground I found I had no need to downshift at stoplights, and could start off and stay in the 11T cog. Despite doing this, that ($7) cog lasts about 1000 miles instead of the 50-200 you'd expect from pulling that shenanigan with a rwd mid-drive.
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Wear is reduced on the entire drivetrain. My green cargo bike just crossed 3000 miles and it is still on its original KMC e11 chain (and steel cluster). It still shows good on the wear gauge.
Improved, distributed traction provides benefits in all types of terrain. Its the most obvious to the outside observer in deep sand, snow and mud, but to the rider the appreciation of the benefit on basic pavement is instant. 'effortless' is the best way to describe the feel of it.
And another: I have a load of gravel to pick up again today. 100 lbs. Distribute some of the pulling load (traction) to the front wheel and the amount of pain and suffering the entire drivetrain goes thru to get the job done is markedly improved. I'll be taking a steep hill that a geared hub would never survive, 2wd or no.