running on one engine

Is there anything wrong with running on one of your twin engines to save fuel?

nothing wrong, just alternate the motor next time you run on one, to keep the hours and maintenance intervals about the same.

Maybe or maybe not depending on the drive system. On a twin shaft system it will depend on your gearbox and shaft seal setup. Some gearboxes don't like having the output shaft turned when the input shaft is not turning as the oil pump is driven off the input shaft. Many shaft seals are lubricated by a pressure water feed from your engine. With the engine stopped there is no lubricating water and you will toast your seal. On the other hand, some gearboxes and shaft seals may be fine, and most stern drives should be fine as well.
Final comment is that the fuel saving when dropping down to one engine might not be as much as you think, and if you are running at anything much above your hull displacement speed on a single engine you may well be overloading the running engine.