Multiple Render Borders in Cycles Preview Is it possible to specify multiple disjunct render borders?



I realize that this is trivial when rendering but it would be very useful when previewing on a less-powerful computer.  I have two complex meshes on separate sides of the scene and would like to preview them both at once.
Unfortunately this is not possible currently, you can only have one renderborder per 3D view.

However, you can have multiple 3D views, each with it's own renderborder:



Note that you can only have one camera renderborder which is global for all cameras, so this is only possible in viewport render.