Hello Mark,
If you load a stereo rank of samples in mono (via the "Organ | Load organ, adjusting rank audio memory options" screen) then Hauptwerk will pick the loudest channel from each sample, and automatically adjust its level to match the mean of the sample's two original stereo channels (so as to avoid any potential for phase cancellation).
However, if you load a stereo rank of samples in stereo but route them to a mono audio device channel (by selecting a mono audio device channel for a mixer bus) then the two stereo channels will be mixed together equally in real-time solely as they're sent to that mono device channel. (If you have any sends to master/intermediate mix buses enabled from the mixer bus then the sends will retain the original stereo field, even if a mono audio device channel is selected for the bus.)