Thanks to oour friends on the 921 team, they have sent me this handy tip, it's a special "safe mode" for the 921 which allows you to output 480i and 480p at the same time.
Just a word of warning here - the 921 safe mode isn't designed to be a mode for the 921 to operate in for extended periods of time. It's meant to allow recovery from various error conditions.
And before anyone asks - no, you can't output 1080i/720p and 480i together in safe mode. Only 480p/480i.
This procedure is described in the 921 manual. The manual also states that pressing the HD/SD button on the remote for three seconds will also achieve the same thing. I tried it and it didn't work. I haven't tried pressing the button on the front of the 921.