Contrary to popular belief, rabbits and guinea pigs should NOT be paired together or live together in the same hutch, for a number of reasons:
Rabbits and guinea pigs need different foods to meet their different dietary requirements; guinea pigs need a diet rich in Vitamin C or else they can become ill. Rabbits are the opposite and may become ill from too much Vitamin C or a lack of other important vitamins. The 2 animals sharing the food may make it difficult to know which animal has eaten what.
Rabbits and guinea pigs communicate differently so it is in both of their interests to have a friend that talks the same language. You wouldn't want to lead your life living only with a gorilla for company - you might get on but could never understand each other properly!
Rabbits are a lot stronger than guinea pigs and can easily hurt or even kill a guinea pig without meaning to, simply by kicking out with their powerful back legs or jumping over the guinea pig. The rabbit may also bully the guinea pig. Sometimes guinea pigs ingest rabbit fur which can cause them problems.
In the wild, rabbits run fast and free, covering the equivalent of 30 tennis courts daily. Guinea pigs have little legs and despite needing lots of space and exercise themselves, it differs greatly to the type of space rabbits need to hop, run and jump.
Rabbits can pass a bacteria onto guinea pigs, which can cause respiratory disease in them.
These differences aren't to say that you can't keep both types of animals in the same household, but keep them separate from each other and pair up with an animal of its own kind. If you currently have a guinea pig then get him or her a guinea pig friend. If you have a rabbit then get another rabbit - it's a myth that getting a guinea pig friend for a rabbit is cheaper!
For more information please see: Why every rabbit needs a friend.