this may be the same ones i read about a week or two ago.. weird and cool at the same time...
it does happen and both babies do survive.
In 1992, Taylor and Evan Barth, conceived 4 weeks apart, were born in Hawaii, USA to Michelle and John Barth. Taylor, the one conceived later, was born first.
In 2007, Ame and Lia Herrity, conceived 3 weeks apart, were born in the United Kingdom to Amelia Spence and George Herrity.
http://en.wikipedia.org/wiki/Superfetation#cite_note-2
In May 2007, Harriet and Thomas Mullineux, also conceived 3 weeks apart, were born in Benfleet, Essex, UK to Charlotte and Matt Mullineux,
http://en.wikipedia.org/wiki/Superfetation#cite_note-3
Todd and Julia Grovenburg of Fort Smith, Arkansas received international media attention for Mrs. Grovenburg's conception of an additional child while already pregnant with a child conceived two and a half weeks earlier. If it were possible to carry both children to term, the birth of the first child would be expected in December 2009, whereas the second child would be due in January 2010.
http://en.wikipedia.org/wiki/Superfetation#cite_note-5 Grovenburg's obstetrician reported that cases of superfetation "can only be confirmed after delivery by chromosomal and metabolic studies on the baby."