Thanks for sharing these fascinating reports, guys! No doubt, as old mcdonald says, many things like this go unreported.
It's not too difficult to envisage species with bicornuate uteri i.e. 2 fairly self-contained "limbs" to the uterus, like sheep and goats, to empty each horn separately some hours, or even a day apart.
When the distance apart is the length of an oestrous cycle, it is techically known as Superfetation
[http://en.wikipedia.org/wiki/Superfetation] and very rare in farm animals.
When I was working as a vet in Yorkshire, a pig farmer showed me a sow that had given birth to a single piglet, then exactly 3 weeks later gave birth to several more - all healthy. Superfetation is very hard to explain in a sow, where a fall in ovary progesterone plays a key role in farrowing and the same hormone is essential to the maintainance of pregnancy.