CMiska,
You are thinking of "superfoetation". Here is ashortarticle that I recently wrote on double pregnancies inrabbits (superfoetation).
Double Pregnancy (Superfoetation) in Domestic Rabbits
Pamela Nock
Rumors persist that double gestations are common in rabbits,yet in the many years of breeding rabbits and doing double breedingsover the years, I've never seen this happen. I've also noted that incases where the buck and doe were housed together, and the buck wasremoved before the doe kindled, she did not produce a subsequentlitter. Does do become immediately receptive after kindling however, sothere are many cases of litters produced 31 days after the first litterwas born.
The fact that rabbits have a double uterine horn does not mean they are meant to carry a double pregnancy.
Most animals have double uterine horns including dogs, cats,horses, pigs, cows, etc. Litter bearing animals such as rabbits, pigs,cats and dogs have longer uterine horns than do animals such as horsesand cows which may only produce one or two young.
Conversely, humans (like all primates) do not have doubleuterine horns (simplex) or extremely short uterine horns. However,there are at least 25 documented cases of "superfoetation" (doublepregnancy) in humans.
While a double gestation in rabbits would be extremely rare, itis documented in European Hares, which can conceive again in the finaldays of the first gestation.
Sources:
Rabbits & Hares
"http://www.shsu.edu/~bio_mlt/Rabbits.html"
Domestic Rabbit Reproduction
"http://www.fao.org/docrep/t1690E/t1690e05.htm"