Cron,
I can see your problem and I will try and keep it simple for all the others
Helicopter performance is affected by weight (mass), altitude and temperature, the WAT limits. Look in the flight manual and depending on manufacturer you could have several charts that give you the maximum weight for a given density altitude (pressure altitude versus temperature).
This weight would normally be the maximum for All Engines Operating (AEO), but there will be other charts for One Engine Inoperative (OEI) performance. This MAUW should give you enough performance to make an safe, comfortable approach and landing. Depending on aircraft this could be a running landing or an approach to a hover.
The second engine comes into its own during the critical phases of flight, take-off and landing. If you are at or below OEI MAUW and you are using the correct techniques, should a power unit fail you should be able to land safely.
For an explanation of proper techniques, Performance Class I, II and III and Category A operations have been done to death before (but is still confusing to those who have not done it, and to those that have!)
The answer to your questions, yes and no, yes and no, yes and no, yes and no. Clear?