You have two independent sentences here.
- In the past six years I’ve written twenty short stories.
- About half of those [stories] have since been accepted to literary magazines or paperback anthologies.
In a paragraph, they would look like this:
(A) In the past six years I’ve written twenty short stories. About half of those [stories] have since been accepted to literary magazines or paperback anthologies.
When you put them together, they look like this:
(B) In the past six years I’ve written twenty short stories, and about half of those have since been accepted to literary magazines or paperback anthologies.
Yes, sentence (2) is syntactically complete and is therefore an independent clause in (B). It can stand on its own - see (A). In (B), the independent clause has a demonstrative pronoun as a subject and then a predicate.
The "since been" does not affect the grammar of the sentence, even if it raises a logical question "since when?"
On a different note, I think the use of "since been" is problematic. You wrote the stories within a period of 6 years. Does "since been" refer to the end day of the six-year timeline or does it refer to each of the days when you finished writing those stories? Is "since" important information? If not, I feel the sentence would be better without it.
In the past six years I’ve written twenty short stories, and about half of those have been accepted to literary magazines or paperback anthologies.