
Talking to people in the USA re reasons to do it, one was that you might really want a litter from your bitch,a dn timing was crucial.
Your choice of stud was perhaps an older dog who was no longer very fertile, and you would use the second younger dog as a back up at the end of the fertile period (giving time for the first dog to fertilise eggs).
This would of course require accurate progesterone testing and timing of the matings.
What you have to remember the window from breeding from bitches compare to males is quite small.
If you have bitches that come into season every 9 or 10 months, a miss or two and you have seriously dug into your time, especially if you have waited to breed from a successful show or working bitch.
I have had a second litter from a bitch over two years after the first, and there were only two season between the seasons she had the litters on, one of which she had missed on.
Had she already been a 4 year old bitch first time this litter would be it as she would be 6 and I might not be able to fit another litter in. This is a bitch conceived abroad to bring in new bloodlines and the litters are designed to find out which lines she will blend with best, so no point just having one litter, which didn't have what I wanted (best quality in wrong sex).
Breeding from any dog should not simply be about putting a dog to a bitch and getting puppies, a lot of thought has to be put in to the bloodlines used and their effect on the bigger picture.