I firmly believe that Centaurs womb is on the horse end. It just makes the most sense for delivery. They’d definitely only have one set of ovaries and a womb.
As far as delivery of sperm; male centaurs dicks would be significantly longer then a humans. So, a male centaur wouldn’t have any problem reproducing with a female centaur (barring any other reproductive issues).
A male of other races (humans, orcs, etc.) might have issues with the sperm actually being able to be delivered to the womb.