I think I finally have some of this figured out and just had to tell someone. All of the Shruthis are going into a MIDI patchbay, along with the four MIDI outputs from a Roland A90 controller. MIDI out 1 from the A90 goes to the Ladder Shruthi. MIDI out 2 goes to a MIDIPal running the Dispatcher app, which then goes to the other four Shruthis. MIDI out 3 goes to a PreenFM and MIDI out 4 goes to an Alesis IO Dock. External Zone A on the A90 is for the bass and controls the ladder. External zone B is for the rest of the keyboard and controls the other 4 Shruthis. External zone C controls the PreenFM, also for the rest of the keyboard and External zone D controls the iPad.
The ladder is easily thick enough to provide lots of bass coverage, and then I can layer the Shruthis with the Preen and iPad synths. It sounds really cool. The neat thing is that each of the four Shruthis has a different filter board so there are subtle (and not so subtle) differences in the notes in a four-note chord. This is fun!
Sorry, I just had to tell someone.