A rabbit and a guinea pig mating would be like a human and an ape mating, it's not possible.
They are similar in a lot of ways, they can live together harmoniously, most of the time, but they cannot breed.
I do have to say this, spaying and neutering is possible and recommended for rabbits. It not only prolongs their lives but it makes them easier to handle and more pleasant. It's not that expensive and it will make for a healthier and happier pet.
I'm not sure if Guinea pigs can be spayed or neutered but I can't see why not.
Talk to your vet. This behavior may lead to problems down the road, best to get it under wraps now.
