I've done it, I have had three RPs (C&C, Hackers and Dazzling, on the old site and they're being abbreviated now) in the same time with the same concept and technology but all with different plots. I've had two crossover (C&C and Hackers) but it was then done so that the Dazzling players soon migrated over to C&C itself due to it being more active and necessary for a plot. I enjoy doing it but it takes a lot of work and dedicated players. But it can be really interesting and enjoyable.