If for any unforseen reason you have to fly very well below your optimum flight level, i.e. depressurization, it might be interesting to consider to shut down one engine (twins), only if you are dangerously running very low of fuel to reach any airport. It your flight planning was serious enough it would never happen.
Specific range for a jet aircraft depends not only on Fl, GW , etc but also on engine efficiency, if you HAVE TO fly at FL100 at long range, corresponding LRC speed will be so low that both engines will run almost at 70% at this rating both engines will be very ineficcient, in this scenario you may retard one throtel and check for new specific range, if dessesperated even shut it down.
For instance a MD80 flying at FL 130 and GW 48 T, specific range for both engine cruise is 122 NM/ ton of fuel, while cruising with just one engine is 130 NM / ton.
It is really not much difference for a real scenario for killing one engine, but at least you can go a little further. Anyhow, Would be we able to fly it precisely? What about the trim drag?
Too much risk for such a low gain.
REGARDS