Well considering my SR+ would slow down to creep mode (that i have enabled) going down a fairly steep hill unless you press the accelerator, I'd say you don't need 2 motors?
Maybe the declaration would be sharper with 2? But I think a single motor can quite easily recover all it can from forward motion.
I also think it's not linear. I noticed that deceleration from going 60+ is considerably less than from going 30 or less. The car lets you gently decelerate at higher speeds and more like brake and lower speeds.