The PCF 8574 would be connected to one of the Tivo's i2c busses and using, initially, Thomsons i2cset4 script to set/reset the outputs of the PCF chip to operate a relay to switch between outputs of two Tivos. The script could be invoked using a trickey sequence.
This would enable remote selection of the Tivo required for input to the viewing TV.
The circuit could also be used to control any other external device using a relay to switch this device on and off (eg room lights) wth the action commanded from the remote (trickey) or a cron job or web (tivoweb remote control).
With further refinement the same device can be used for input to the Tivo from the outside world ie. record start/stop alarm conditions or whatever.