Hello.
It sounds like you plan to break the 18 servos into smaller groups with a voltage regulator powering each group of servos. You can do this on a single 24 channel Maestro by breaking up the servo rail into separate power banks. Brandon explains in this forum post how to do this. Although I have not personally tried it, I do not see any reason that powering each group of servos from a separate voltage regulator would not work, and you should not need any additional circuity.
- Grant