Two breeders or two individual rabbits? Chances are they are line bred. It's considered perfectly safe to breed aunt/nephew, or even grandsire/granddaughter and so on. It's usually not a great idea to breed daughter/son to dam/sire, or siblings unless you have something in the bloodline that you are trying to pull out (for example chocolate or dilute (blue)). In the event you breed that closely be prepared to cull heavily so that you only keep the best for breeding. Anything less close than direct family you should be fine to breed, just try to select rabbits that compliment. If you have a rabbit that's a little weak in say the shoulders but has a fantastic rise and hindquarters, try to find a mate for it with outstanding shoulders and so on. Hope this made sense.