I know that there are people that do house them together but that doesn't mean that it's right, safe or healthy. This has been discussed numerous of times on here and on other forums I'm a member of and the majority agree that they should not be housed together. Even though the bunny may love the piggie, they do get jumpy and can accidently injure &/or kill the piggie. There's also a bacteria or virus that the buns can carry that is very dangerous to the piggie. (I think it's pasturella but I'm not 100% sure that's the name of it.) Also, pellets are different for each. They both get the same types of hay and most veggies, but the pellets are formulated for each specific species. Therefore, most responsible owners will separate the two.
As JennG said, they have different dietary needs. Also, rabbits can jump or kick and accidentally injure (sometimes fatally) a guinea pig.
They may be "best of friends" but they shouldn't be housed together.
Yes, I can see how it would be a problem.
And it's not that I was an irresponsible owner. I was 9 or 10 at the time so I really didn't know any better.