Yes, JTs quite correct, but while the requirements are straightforward the reality is not. In older twinjets, (I refer in particular to the 1-11 because I know it) complying with the legal requirements stated by JT above would have imposed an unrealistic weight peniltly. It was not uncommon to have a single-eng stablizing altitude in the very low teens sometimes lower. Hence the reason why the charts I refered to earlier were produced for approval to opperate in/over mountainious areas.
Going back to reality for a mo, re-routings flying lower than you might was/is of course common, the trick was planing for the worst knowing where you are and what you were going to do should you lose an engine, this was of course part of the en-route briefing. Having said that your risk exposure, when considering the alps and the 1-11 was not unreasonably high, it just needed a bit of thought thats all.
Best regds