Okay, this one is about cooking and some folks say it is also a metaphor.

Let's take the cooking part first.
Huck is saying that Widow Douglas cooked all the food separately and placed it on the plate separately as well. Now, when you cook, say a stew, you would put all the ingredients together in one pot (the meat, potatoes, carrots, onions (and all the odds and ends you have in your refrigerator) and so on) the flavors all combine to make it a better dish. And you serve it that way too. All the juice mingles together and you have great sauce for your bread!

Now for the metaphor.
In the story the races were segregated and the whites much more superior. Twain is hinting that if we combine the races and "all get along" so to speak, we will have a much more flavorful world and things will go better.