I understand all of the challenges here.
As already figured out, 23 zones requires a minimum of two Iro2s.
At this time they don't 'talk' to each other. To do so, it will require the Iros2's 'engine' (internal calculations) to be expanded from keeping track of 16 zones to keeping track of 32 zones. It's just like 'bit' expansion in the computer world. Right now the Iro2 is an 4 bit processor (for example). You need a 5 bit processor to keep track of all 32 zones independently of each other. Roughly.
All is not lost, though, if, the maximum watering time for the first 16 zones is never more than 12 hours. If so, setting the Iro2 to start at midnight, and setting it to full Auto, will result in watering being done by Noon.
Then, a second 16 zones can be \set to start at Noon. It too must be finished with all watering by midnight in order for full Auto mode to work properly.
If you have a residential yard, even a large one, that the first 16 zones can be always be completed within 12 hours, and the second 16 zones can be likewise completed in 12 hours, then you're done. The Iro2 will schedule perfectly without fail and without incorrect load on the source water connection or pump.
This is the challenge parks have and why in most cases city parks cannot use Iro2 units. Many times the parks are so large that a 'day's' watering can't be done in 24 hours. And thus a more sophisticated 'engine' that can keep track of the accumulations past a 24 hour boundary. This is what the commercial units indeed do.
I don't know if Iro2's 'engine' is being modified to go past 16 zones. And if it is, I further do not know if the 'engine' is being modified to allow for a 'cycle' to got more than 24 hours. If so, for both, along with an Ethernet connection, the Iro2 will appear very attractive to municipalities.
WRT to wiring:
Your easiest solution is to stay within your wiring's constraints. I would purchase a 16 zone, an 8 zone, and another 8 zone unit. Three transformers, three return wires each in the correct cable. You'll end up 'wasting off' 4 zones on the 1st unit, 2 zones on the 2nd unit, and 3 zones on the 3rd unit. I think this cost is less that the cost associated with an isolation system otherwise needed to accommodate your 3 cables.
The reference Rachio refers you to is correct and accurate. And if you don't follow it you will probably burn up your transformers (not your Iro2 units though).
This project is within your ability if you implement 3 Iros2 units as I described above. And that you can do all watering needed of the 16 zone unit in 12 hours, and all watering of the 8 zone units in 6 hours.
If you stay with your 16 and 8 zone controller, you need a commercial electrician to guide you. This is not an obvious setup and most residential electricians will fail.
This is a problem that comes up in the HVAC world. Multiple transformers that end up being put in parallel to each other and thus burning themselves out.
Go get three Iros - 16 and 8 and 8 zones - and keep the project easy!
This recommendation also provides to you some expansion capability per cable: 4 zones for cable 1, 2 zones for cable 2, and 3 zones for cable 3.
And then when/if the '5 bit' Iro3 'engine' becomes available, you'll discard the 3 controllers in favor of a fully automatic 24 or 32 zone unit. And maybe you'll get an added bonus that allows for watering to go past 24 hours per controller cycle.