The ONLY two microcontroller project I've ever done was not needed for processing power, but for redundant control (2 heads are better than 1) for a railroad switch position detection and/or bridge in place detection. Both processors read 4 proximity transmitters and decided which position to display. They then conferred via the SPI port, and if in agreement, toggled the input to a switching power supply controller to turn the proper output on. This was a vital function, one that could not give a false positive, so if either processor hung up or quit working, there was no output.
The processors gave error indications for open or shorted proximity inputs, and once a second even had hardware to open and short the inputs to make sure they could detect those conditions.