Hello Jonathan,
You can only have one audio 'device' selected at once in Hauptwerk (to ensure that all channels are perfectly synchronised, otherwise audio glitches could occur). However, you could use macOS's "Applications/Utilities/Audio MIDI Setup" to create 'aggregate' devices, for which macOS handles synchronisation and presents the channels of each aggregated real device as channels on the single virtual 'aggregate device':
https://support.apple.com/en-us/102171
Then you could select that 'aggregate device' as the output device in Hauptwerk.
However, I'd be surprised if a Mac's built-in output has a bass response extending deeper than a professional audio interface (such as those from MOTU).