Happy to help. Basically, I connected the first Iro using the instructions on the website: http://support.rachio.com/article/360-rainbird-wr2-wireless-rain-sensor. There are four wires shown by the green arrows in this picture (green to S1, white and black both to SC, and red to SP):
I combined them by adding an additional pair of wires between the two controllers, shown by the red arrows. One of the wires goes to S1 and the other to SC. They are then connected to the second controller (S1 to S1, SC to SC) as shown in this picture:
And that's it. Works like a charm.