So I have had one AIU (and it's connected switches) running for a couple of weeks now; but had to wait for some 6 conductor #26 flat cable to arrive, before I could hook up the 2nd AIU. Well it arrived yesterday, so I built up a 16' long interconnecting cable. Way easier to do this than making up cat5 network cables I found. Just strip back 1/4" of the jacket, slip on a connector, one fast crimp and you are done. No tedious wire unravelling and aligning as with a cat5 cable. 
The 2nd AIU works great and so far the connected switches seem happy as well. I used 10 conductor and 15 conductor #22 multi-conductor cables to run to two switch terminal strips several feet away from the AIU. Multi-conductor cable greatly reduces the wire clutter under the layout, which is a great thing!
Then I robbed an idea from Susan Deats' site and "siamesed" 3 pairs of switches together, so that in each case one AIU switch channel operates 2 switches together. This reduces switching chores and allows more switches to be connected to each AIU. Nice idea Susan! 
So based on a statistical example of ONE, I can say that the extended 6 conductor #26 flat cables between AIU's seems to work fine. And this means you can put each AIU in the approximate geographic center of the area that it will serve, just like when you are using more than one TIU. One thing, I wish I could have found SOLID #22 multi-conductor cable locally, rather than stranded for the switch hookups. I found you have to tin each wire end before inserting into the AIU connectors. Otherwise you have "cat whiskers" all over the place, just waiting to cause trouble! Solid wire would have been much easier to deal with.
So this might just prod along anyone who is still on the fence about AIU installation. Once you get your head around it, it's not that tough, and if you are thinking about moving to wifi operation one day, you will be ahead of the game. One thing for sure, I wish there was a machine that you just stick the cut-off end of your multi-conductor cable into; and it would automatically strip 2" of jacket off, then unravel the wires, strip 1/4" of insulation off each, then tin each wire end. Wouldn't that be cool! 
(OK, it was a nice fantasy!)
Rod
