It is best to avoid mixing rabbits and guinea pigs together as guinea pigs can get some diseases from rabbits and they can bully eachother.
Both rabbits and guinea pigs are social species so companionship is an important aspect of their care. You may therefore consider housing rabbits and guinea pigs in a pair or group of their own species.
Please take care to avoid any unwanted pregnancies or litters by separating rabbits or guinea pigs of the opposite sex.
Note: If mixing is absolutely unavoidable, it is best to try to mix guinea pigs and rabbits when they are both still young. This way neither animal will feel that its territory has been invaded and they will be less likely to fight. It is generally not recommended to introduce a young guinea pig to an adult rabbit.