How are your controllers wired now? Are valve commons (C terminals) connected together?
S1 terminals are inputs, SC is actually the terminal sourcing AC voltage.
If your commons are tied together (there is no meaningful AC voltage between the C terminals of the two rachios). Than you can try running a wire from S1 terminal of one rachio to S1 terminal of another. Set them up as Rain sensor and make sure that neither rachio shows that rain sensor is activated. If everything looks to be working so far, than you can connect that wire via the rain sensor to SC terminal of one of the rachios.
Warning! Only attempt this if your Rachio units share a common wire to your valves. The wire between SP terminals is optional. I'm not sure if you will end up needing it.
Note: This setup is prone to noise on the wire. When it rains Rain sensor will disconnect and leave the two inputs floating. Flawless operation is not guaranteed.
Perhaps @emil may provide better details on how the rain sensor data is interpreted. From what I understand, as soon as rain sensor dries out rachio will resume business as usual.
Having never used a rain sensor I do not know if, for example, automatic schedule will be skipped in the evening if rain sensor got wet during the day (but has since dried out).
Because of this I've never connected my old rain sensor to my rachio (even though I had one available). In reality you are much better off using an actual rain measurement from one of your local weather stations. If you don't have a good one available, you can try searching Weather Underground and porting the data via API.