Exporting tracks mono vs split mono vs stereo

I'm sure I've done this easily before but I'm having a brain fart apparently. I'm busy exporting stems for someone else to mix. I'd like to export the tracks that are stereo as single stereo files (e.g. organ, piano) but the tracks that are mono as mono files (e.g. lead vocal, bass). If I select "mono" the organ is summed to mono, if I select "stereo", my lead vocals become stereo and if I select "split mono" my organ gets split into L and R.
 
Where am I going wrong? This doesn't seem like rocket science.

Well, it's only slightly confusing 
 
When someone says "stems" to me, i think of a subgroup of tracks, like drums or vocals, keyboards or guitars, so in this scenario, one would assume, quite rightly, that the export source would be a buss which by it's very nature is stereo.
 
What I think you're doing is exporting individual tracks so if can clarify exactly what you're doing then a solution will I'm sure present itself.