Reproductive Issues in Pets

A Delicate Question About Doggy Reproduction.

Recently I saw my neighbour dog, who is in heat, standing on the street in the butt-to-butt position of post-copulation, with the male dog who fathered her last litter of puppies. I was walking my dogs at the time I saw them. Not 5 minutes later, not even 6 mins but 5!, she came up to my male dog and boom, they copulated, too! My question is this: now that she has the spunk of two males in her, which one will father the litter? Is it possible for it to mix&mingle and both will father some of the puppies (if she gets pregnant). I could not find any answers online but I also didn't really know how to phrase my question either! :)