rabbits and guinea pigs as friends

Is it possible for them to get along as friends? currently I have my piggie cage as an open cage. Will they be safe if I get a new bunny companion in my home? Will I need to build a cover for their cage? I dont plan on giving a new bunny unsupervised free range time in the home until I get to know him/her, but are there things I should consider having piggies and bunnies in the same house? Are there illnesses that transcend from piggies to bunnies and vice versa?