I had not thought to add this. You can do this now by taking measurements of the drivers when combined in series and entering that as a single driver. This does make the spatial option unusable.
I'll look into it. It can be done, I'm not sure how much it will take to do that, but a quick look into the code makes me think it may not be too difficult. I can't say when I might get to this. If it looks like a short bit of time, I may do it soon. I can't say more than that for now.