There should be no problem recharging in 9 hours if plugged in parallel while charging. I usually get home at 20:30 and by next morning at 07:00 they are charged when I leave again. I admit I never got up to check outside to see if they are done at 04:00 or 05:00. In your case, if you could plug your bike and recharge them both in parallel I'm sure you would always leave with two fully charged batteries. Thus you would discharge both of them as little as possible and stay above a 75% discharge most of the time.

I'm wondering about 36V myself and the range it could get. I know the speed and torque would be increased some. If I could plug my bike at work to recharge I would do it. I would need to get a 36V charger tough.