Check out this photo of one of our user’s installation. This user wanted to maintain its old controller side by side the new IrrigationCaddy. The wiring (even though the photo does not does it justice), is quite simple. Just wire the zones one to one. So a cable goes from a zone on the old controller, to a zone on the new controller (IrrigationCaddy).
It is best if only one controller is triggering a zone at a time, but even if both are on at the same time the controllers should be ok.