I can think of 2 reasons 2wd is worth it..
1- a very heavy cargo bike that can cause a single motor to overheat and fail.. 2 motors would distribute the heat and make for a redundant system...
2- if you just had to stick to low power for some reason.. 250w areas... 2 motors could double the power output..
But, on a light weight BMX, and not paying attention to the legal power limits, hey.. it's a toy.. so one 9C motor pushed to it's full potential will make for a better all around ride.