It's true that both genre and literary writers often start by selling short stories, but it's because these writers love reading and writing short stories, not because it helps sell a novel. Writing stories will not help you write and sell novels, whether you're a genre writer, or a literary writer, unless you can sell these stories to top magazines. Generally speaking, it's safe to tell any new writer this will not happen.
The only sane reason to ever write a short story, no matter what kind of writer you are, is because you love reading and writing short stories.
Now, whether you consider your first two novels tuneups doesn't matter. Whether you consider your next novel a masterpiece is meaningless. No writer ever gets to decide quality, be it good or bad. Only the reading public gets to decide quality, and editors judge what they buy based purely on what they think the reading public wants, not on what the writer thinks is good or bad.
It's often the writing the writer thinks is poor that sells, and the writing he thinks is great that fails to sell.
But back to short stories. It's heck of a lot harder to sell a short story to a credit worthy magazine than it is to sell a novel to a good publisher, and this is true for two reasons. 1. The competition is much, much, much higher with short stories. 2. Not only is the competition tougher, but your main competition is bestselling short story writers.
You do not have to knock Stephen King out of contention in order to sell a horror novel. You just have to write a horror novel a publisher believes will turn a profit. King's novel and your novel go into different slots. They don't compete with each other. The publisher can buy King's novel, and he can buy your novel.
But you do have to knock best selling writers out of contention to sell a horror story to a good magazine. Magazine slots for stories are always limited, your story is sitting there in the slush, and the editor has just two choices. . .does he fill the slot with your story, or with Stephen King's story? In this case, the publisher can buy King's short story, or he can buy your short story, but he can't buy both because he has only one slot.
When a magazine is credit worthy, every good, famous short story writer out there who writes the kind of story that magazine wants will be submitting stories, and, as I said, slots are limited. You can't be just as good as Stephen King to sell a short story, you have to be better. You aren't competing simply story to story, but his name and fame against your own lack of name and fame.
Nor is it easier to write a publishable short story. Good short stories are not easier to write than novels, they're tougher to write. Shorter means faster, it does not ever mean easier. You may have to write a hundred short stories before writing one that's really any good. Or two hundred, or three hundred. Most who try never, ever manage to sell a short story to a credit worthy magazine, or to any paying magazine, even when they do write hundreds of stories.
Why do you think short stories sold to good magazines make good credits? If selling one were easy, it wouldn't be considered noteworthy. Short story sales to good magazines are considered good credits solely because such sales are incredibly difficult to make. A editor thinks, "Wow, this guy managed to sell a short story to X magazine? Man, not many can say that! Now, I wonder if he can write a good novel?"
The answer is not always yes, he can write a good novel. A writer can spend many years learning to write a publishable short story, only to find he can't write a publishable novel. Many even find that the forms are so different that learning to write publishable stories means they have even more trouble writing a publishable novel.
Two years is a long, long time, if you're good, and if you're prolific. No amount of time matters if you aren't any good, or if you putter around, taking forever to actually finish and submit your writing.
If you want to use these two years well, write your ass off on novels. Unless what you really want to be is a short story writer. Then, and only then, should you spend time writing short stories.
