No. It would actually be faster. The real wonder of 2wd ebikes is the benefit of distributed traction. Among other things, the fact that both wheels are pulling you forward makes for some synergy that is just not possible when powering only one wheel. Your acceleration will also feel smoother and more effortless, for lack of a better term.
I have done quite a few 2wd bikes, including twin hub and mid-drive+hub. I did a write-up on the topic here.
I like to build top-quality-component ebikes from the frame up. Quite a few of them are dual motor or AWD or 2WD or whatever you want to call them. Why would you build an AWD ebike?
My most recent 2wd is sort of a v2.0 of a Larry vs. Harry Bullitt. v1.0 is described in the series above in detail. This one is geared for the steepest of hills. Last weekend I did a run on Saturday that carried 100 lbs of gravel home from Home Depot, up over a very steep grade. I did it again on Sunday (a bag of gravel seems to cover about 1/4 of what you think it will). I took this pic on the way to Home Depot and I made it thru that deep sand. You will never get through deep sand by adding more power to the rear wheel no matter how much power you add. The front wheel will submerge without some power to keep it up on top.
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Also, this is my for-reals sand crawler. I take this along a deserted stretch of beach where if you lose it, the walk back pushing the bike is probably enough to kill you. Its 4 miles to the next inlet into shore that a bike can get up. Sand at this spot is moist and you can ride on top of it anyway, but that is only cuz this is a spot where a river overflows to the sea.
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