Hello Greg,
Whilst it's certainly possible to configure Hauptwerk to send a stereo aux mix-down simultaneously to a separate pair of channels (e.g. for headphones), I would instead recommend just configuring RME's TotalMix control panel mixer to mix all of its analogue output channels simultaneously to its headphones output channels (9+10). That's likely to be more convenient since it would then work for all applications (e.g. sequencers, music players, and Web browsers), rather than just for Hauptwerk. That's how I have mine RME UFX configured.
RME's documentation covers how to use TotalMix in depth, but basically, within TotalMix, you should just need to go to 'Function | Matrix view' then click on cells in the 'PH 9/10' row to add virtual connections (sends) there for each of the desired 'Out N' columns. (Also make sure that the 'PH 9/10' fader is turned up in 'Function | Mixer view'.)