The Rover Group recently made a trade off for having either one larger rover or two (or more) smaller rovers. The option with two or more smaller rovers proved to be more reliable and cost effective for this particular mission.
Later it showed that having more than two vehicles would again significantly add complexity to the system because of the required level of miniaturization that would have to be applied. Therefore the design is limited to having only two operating rovers.
Both rovers will perform the same tasks during the mission; The vehicles will not be customized to perform separate tasks, potentially speeding up the mission. The team rather was in favour of the additional reliability obtained by having two identical rovers both capable of completing the mission on its own.