You can connect the rain sensor to the master controller, and if the other controller is set as ‘remote controller’, then the rain sensor status will be applied to all stations, including master and remote controllers. Rain sensor events will also be logged onto the master controller.
OpenSprinkler DC has comes with a pre-installed SD card so you can set it as master controller and it should log data onto the SD card. Of course you can also set OSPi as the master controller.