Lots of good advice above. I have been building and flying multiengine planes for many years. Here are the main points:

1. Multi's fly the same as an equivalent single, if all engines stay running.

2. There is no such thing as a multi that will not lose an engine eventually. If someone says there is, they just haven't flown it enough yet!

3. Flying with an engine is not hard, but it is different. Recognizing when one has failed and dealing with it properly is a unique skill that must be learned and practiced.

4. Most scale multi's don't last long because they are so darn heavy. Vast majority of scale planes are way way overweight for their size, and often underpowered too. Because of the heavy wing loadings, most scale multi's would be scary to fly even if you removed the engines and mounted one big one front and center.

5. One simplifying technique to handle the loss of an engine is to simply throttle back and glide in. Treat it like a dead-stick with a single. Since most scale multi's have such a high wing loading, they glide like bricks! So not really a practical solution. And there is still the issue of being able to recognizing the engine failure before losing control.

6. Last, the engines don't need to be synchronized. Set each mixture individually for reliability (not too lean), and give a good long nose up at full power test before takeoff to verify adequate mixture.

Definitely get a cheep rugged twin trainer to learn multiengine flying. Setup each throttle on a separate channel and use the programmable mixing of a computer radio to slave them together. There are many ways to do this. This way you can throttle one back during flight, way up high where it is nice and safe, to learn engine out flying. Just get some cheep ARF, your (anyone's) old forgiving trainer is just fine, and bash 2 engine boxes onto the wings. Nothing fancy, then go for it. This address basic engine out skill.

Now learn this again with high wing loading. Just add weight to your twin trainer a little at a time until you can fly it engine-out at high wing loading. repeat until the wing loading gets as heavy as your proposed scale monster.

When you can do both of these, then your skills are getting into the ball park.

Multiflyer