@prob, good afternoon, thanks for reaching out. Great questions! I'll answer them below:
First off, could we combine any of the 18 zones together into bundles of two? If you have small zones with the same zone attributes (vegetation, soil, nozzle, sun/shade, & slope), we might not need to setup two controllers; i.e. if you have 4 drip zones, we could combine them into 2 zones and fit all of your wiring into a 16 zone Iro. If that isn't possible, please see notes below:
Correct. Will you be installing your Iros indoors or outdoors?
We'll need to split the zones with their related common wire. Therefore you might have 12 zones on the 16 zone Iro and 6 zones on the 8 zone Iro. You cannot cross wire the commons between the two Iros. This is why I'm hoping we can combine four zones into two zones and eliminate the need for the second Iro.
Please see #2 (above).
Hope this helps