Surely no more than sound judgement is the arbiter here? If a single engined engine ferry can be carried out safely why not? As to carrying out an operational task in a one-donk twin then I'd want to know if a war was on before I thought seriously about it...
I had a starter quill failure in an As355 once, fortunately on a site on cliffs that subsequently, and very publically, fell into the N Sea. Leaving pax behind it was possible to hover taxi on one donk over the cliff and attain translational lift, flying away to refuel rotors running en route to base. Took a bit of cushion creep, but organised in advance, so what? Just had to realign my head to regard the twin as a single - so?