[gpm]

Dual license means that you are offering two separate licenses to the code, and that people can use the code under either license.

So if you wanted "MIT unless you're FANG, in which case you get an even more liberal license", you could do that with dual licensing. On the other hand if you want "MIT unless you're FANG, in which case all rights reserved" you're going to have to come up with some custom license.