You should be able to have the 16 zone Rachio replace both of them. The high level description is that while each of those current controllers run a number of zones, you can plug all zones into one Rachio controller.
For the wiring, when you take off the face plates of both those existing controllers you will see wires that plug into the zone terminals (usually marked 1-X). So one of them will have 8 wires that correspond to your front yard and the other will have 4 wires that correspond to your back yard. Those are the wires you’ll want to pay attention to as they’ll plug into the zone terminals in the Rachio controller.
You might also find a common wire, I think usually these are white, but I don’t think color is always a good indicator. See if the underlying wiring of your existing controllers call out a common wire and be sure to mark that as well as your zone wires.
There’s an in-depth article here that gives good information about wiring as well.
I had the same old controllers times 2 myself so I can confirm that yes, 1 Rachio is all you need. If you have more wiring questions, post a picture of the terminals on your old controllers so people can answer your specific needs better.
Ok great idea, thanks. So here are wiring pictures from the 2 controllers I want to replace with one rachio controller. The question I have is how to wire them into the rachio since each has its own common wire. Right now each has its own power supply, they are not somehow wired together.
Your setup looks pretty standard, just like mine. The rachio will have multiple terminals for commons. Just plug each of your 2 commons into any of the rachios commons, it doesnt matter which one. You can even put the 2 commmon wires in the same common terminal, it wont matter. Then each left over wire goes to a zone so plug them into 1-12. It doesnt matter what order either.