Okay first surprised you just learned it. Every single egg can have a different dog's sperm fertilize it.
In rare breeds, AKC allows them to use multiple sires and then DNA test. The reason is to help expand the gene pool faster. Now that there is DNA testing, not sure why there would be a lot of controversy. MOST breeders have one stud they want and not interested in using 2, but it is an option.
Also, they breed at same time (artificial) when the bitch is ovulating. Although there is TONS of speculation about eggs being fertilized at different times, the reality is that generally there is less than 4 days between the first egg fertilization and the last. Sperm can live a few days, so even with one stud an egg can be fertilized first day then 2nd or 3rd. Point being using 2 studs won't be the issue. MANY breeders will claim a small or fading pup is a premie, but I have seen research on necropsy studies saying no… it wasn't premie.
THAT said, I am sure if you have a bitch have puppies early, even 3 days difference there would probably have impact on survival.
Hutchison, D.V.M., who specializes in canine reproduction in North Ridgeville, Ohio, agrees. "Poor timing is still one of the main causes of a missed breeding," he says. "Bitches have a three-week heat cycle on average, but there is a lot of individual variation, and breeding on only three or four of those days will result in fertilization. There really is no magic day. I've seen a bitch ovulate on day eight and another one ovulate on day 32."