Lil Cuck,
Robin Baker, Ph.D. (an evolutionary biologist) notes that this phenomenon is possible and actually happens on relatively rare occasions in the case of fraternal twins, in his book:
Sperm Wars: the science of sex (1996, BasicBooks, 319 pp).
He gives a relatively brief, readable description of how it can happen if the sperm from more than one man is present in a woman at the time of ovulation / conception.
The above is a "science for the layperson" book which unfortunately does not include an index, so although I recall reading Baker's description of this phenomenon I unfortunately cannot give you a page reference. In any case, the above book is based on:
Baker, Robin, and Mark Bellis, Human Sperm Competition
which is the science version. Presumably the latter includes appropriate citations to preceding authors and includes an index.
I note the news article you gave a link to concludes with:
"James Harrison [husband of the twin's mother Mia] has agreed to raise both twins as his own, according to the Daily Mail, and the couple intends to tell their kids the whole story when they are a little older. The couple is also expecting another child in a few months."
That was good to see (re. the twins). Re. Mia's new pregnancy, yes — I suppose a paternity test would be of interest to J. Harrison. On the other hand, maybe it won't be. Perhaps he has agreed to care for and help raise her children regardless of paternity.
—Custer
