Does the supported weight of drywall anchors scale linearly?
This should largely be the case as long as the anchors are relatively far apart.

Think of it this way, two 50lbs. anchors on different walls can each support 50lbs, whether or not the load(s) supported is shared between the two or not.

However, any holes in the drywall weaken it slightly, so while an anchor in drywall may be rated for 50lbs, putting two within a few inches of each other will decrease this rating.

I don't have a good answer for how far apart the anchors need to be, but if you can span anchors to 16" apart, you might as well just mount directly into the studs (assuming 16" OC).