Manufacturers have been toying with similar ideas for a while now, but this might be the most elegantly executed version to date: a detachable, handheld component that nests in the middle of a larger showerhead ring.
This is a best-of-both-worlds design by Thomas Baker, keeping the flow of water from above regardless of whether you leave the attachment in – much more comfortable than stopping the main stream (which has a nice, wide, rain-style spread even with a central gap).
Sustainable or not, it is at least a step up from opposed-head shower solutions that often consume twice the water. In both configurations of use, this combo set uses the same amount of liquid.
The catch, of course, is that it is two fixtures in one and requires slightly more complex plumbing in the background. Perhaps you can pay for the extra pipes by the amount of water you would save over a split-sided system with double water output.