Most people get their Private Pilot Rating in the simplest and cheapest aircraft and then move up to something faster and sexier. The transition is usually not that difficult, some time with an instructor in that aircraft and an oral or written quiz. However, moving up to a twin is a little more involved.
The Multi-Engine rating itself is not particularly hard. After I had obtained Private and Instrument ratings in a single-engine airplane, it took only 10 hours of dual instruction to pass the Multi-Engine Private check ride. About the same when I went back to get the Multi-Engine Commercial rating.
There is no requirement for a written knowledge test for this add-on rating. You just have to show proficiency and that you can handle a twin with its complex and duplicate systems safely. The word ‘malfunction’ will gain a whole new meaning as you will practice failures of all kinds either in the classroom or in the air. In a single-engine airplane, if you lose the engine, you are landing very soon and not necessarily at an airport. In a twin, you have the safety of the second engine to take you to the nearest airport, provided you follow the correct procedures. These procedures are what are important. Some of them you have to demonstrate.
Also, you will get to know the systems in the twin better than you did for the single you took your Private check ride in. How well you know them will be determined in the oral exam prior to the check ride as well as questions asked during the flight check.
For an airline position, multi-engine time is the gold that opens the door to a flying career.