There's really nothing wrong in the kitten growling at the other one under these circumstances. It's an entirely normal process, and you're not going to be successful in trying to train them to "share," as you would with a human child. In fact, in the wild, cats will protect their "catch" from other cats and predators, with loud growls. Your kitten is instictively reacting to a perceived threat, in protecting his prey, e.g., toy from his brother.
You could try some interactive play with both of them with a wand toy, so they learn to associate "fun activities" with each other. Otherwise, don't stress on it. They will eventually work out a "pecking order," as all cats do in multi-cat households.
The only time you need to step in and mediate is if they get into active combat and one kitten is getting hurt. Then separation and "time out" is sometimes indicated. But growling is perfectly within bounds, as is "play-fighting."
