Don't do it. Rabbits and guinea pigs carry bateria in their GIs and lungs that can harm one another. Rabbits can kick and injure guinea pigs, and can also stress them during mating season. Guinea pigs have much different climatic needs than rabbits and should not be kept in an outdoor hutch unless the environment is suitable. Their dietary needs differ; rabbit pellets do not have enough vitamin C for a guinea pig, which can be deadly, and guinea pig pellets are unsuitable for rabbits. Their body language and vocalizations differ entirely, so its easy to have a "misunderstanding" and thus, a fight.
Never keep them together. Either split the hutch so they can see each other for company without contact (which I still don't advise because of bacteria and housing needs differences), or get one or the other. Be forewarned, however, that unsterilized rabbits get along poorly with those of their own gender (especially does).
I may not have ever kept them together myself (I knew better), but many friends and many clients at the clinic tried it, and every time, one or the other gets hurt or killed. In our Small Animal Husbandry class, we learned about it as well, so its actually in the text books that they should be kept apart, not hearsay or wives tales.