Assuming the same amount of motors on a robot of the same weight with the same wheels on the same surface (include same voltage, all the other variables except speeds), acceleration will be dependent on what speed you are geared for. Lower gear ratios provide greater torque, meaning greater acceleration. So when in contest between a shifter and a single speed with all the assumptions made above, the lowest speed (highest gear reduction) will accelerate quicker. So if the shifter robot is in high gear (low reduction) and the single speed is geared for a more moderate speed slower than the shifter's high gear (more reduction), the single speed will accelerate faster. If the shifter is in a low gear that is slower than the single speed (more reduction), the shifter will accelerate faster. That being said, it may not cover as much distance per unit of time than the single speed because the single speed is (naturally) faster, and while the low gear's acceleration is higher, it peaks at a low top speed.

I think what you are referring to is a 6 CIM Single Speed vs. a 4 CIM shifter. By adding motors, you increase the amount of torque put into the drive system, thereby increasing the acceleration. With a single speed, you can optimize your robot to accelerate quicker than a 4 CIM robot at a higher speed. This means that a 6 CIM single speed at 16 ft/s will cover more ground per unit of time than a 4 CIM shifter at 16 ft/s. While this is one use, one of the more popular uses is to use 6 CIMs in a single speed transmission geared lower than your traditional shifter speed, optimized to cover a set amount of distance in the same time as a faster shifter would. Why purposely make your robot slower with more acceleration? With more reduction in your transmission, you will have more torque to push opponents with, yet still be able to get from point A to point B in the same amount of time as a faster robot with less torque, or a faster robot in high gear. However Once a shifter robot goes into low gear, it may have more pushing power than you (though you will still travel farther and faster than it).

Now if you combine the two, and make a 6 CIM shifter, you can accelerate faster and push more in both high gear and low gear, but be wary of tripping your 120A breaker. That being said, the effect of increased acceleration works best within a magic window. The differences in acceleration becomes most noticeable between the speeds of 8 and 12 ft/s, and dwindle down to near negligible as you get further away from the "magic window" (that includes being slower than the range and faster than the range). This means if you want to abuse the acceleration increase, a single speed at ~10 ft/s (or a low gear around that speed) is the best way to get the most out of your extra motors.