Cron, virtually all my helo experience is on singles, but I think I can answer some of your questions satisfactorily:
It may seem illogical, but the power required to fly a helicopter doesn't necessarily increase the faster you go. In fact, if you drew a graph of power required vs. speed, it would look a bit like the curve of a skate ramp - high at the start, low in the middle and high at the end. (This is because of various factors of airflow through the rotor disc and components of drag that we don't need to go into just now).
So, you need a lot of power to hover, a lot to go fast, and somewhat less in between.
If each of your two engines can produce a massive excess of power, then an engine failure at any time should be a doddle, because the good engine will just take up the slack, so to speak.
If the engines aren't up to the task of providing hover power on their own, but need to be working as a pair to do so, you won't be able to sustain the hover if one fails and will have to 'cushion' on to the ground with the other.
However, if you're flying above a certain speed (where power required becomes less), your remaining engine may be powerful enough to keep you flying. What about landing, you ask? Well, in that case, you'd need to do a 'running' landing (either on wheels, or scraping along on skids) so you could keep the speed up right to the ground.
Hope that helps.