Rabbits and guinea pigs have historically been kept together as companions, but this practice is now discouraged as the species have incompatible social needs, communication styles, and disease risks. Each species is better served by same-species companionship.
Guinea pigs housed with rabbits cannot retreat from a much larger animal that may inadvertently or deliberately injure them. The constant presence of an animal they cannot communicate with causes chronic stress in guinea pigs that are highly attuned to social signals. Rabbits also fail to have their social needs met by guinea pig companionship. The welfare solution is same-species, bonded companionship for both animals, not the historically common cross-species pairing.