Have you ever done this?
No, I haven't done this, but I do think it is a fantastic idea. Once the techniques become more reliable (i.e. you can be more sure that there will be puppies from BOTH males), then I would go ahead with this if the normal litter-size is large enough (rather than having one pup from each male, I'd love to have two-three puppies from each).
Is this common?
It's not very common, because most double-breedings do not work and the breeder only ends up getting puppies from one male, but still having to pay for two.. plus the extra costs of DNA testing.
What are the benefits/negatives of doing this?
Benefit:
-get puppies from two males different males in only one litter. So in effect the breeder is basically able to do two completely different breedings on one female, with only making the female go through one pregnancy cycle instead of two. And say you wanted to breed your dog with four different males, you could do two double litters instead of having to do four litters with the female (each whelping and pregnancy IS a burden for a female).
-If you have a perfect bitch and you want to keep her in your lines, crossing her to say 4 good males, you get a wider genepool for your future breedings,
-you have the possibility to take (calculated) risks with one or two of the males, as you'll be able to do the "safe" combinations with the other one or two males.
Negative:
-usually does not work and end up getting puppies from one male only
Why would you do this?
-look above to the benefits. I think two litters (absolute max. three!) is physically enough for a bitch, and I would get the possibility of using more than just the two males.