African cichlids have very different water composition and diet requirements.... if you catered to the new world cichlids requirements, your African cichlids would suffer, become ill, etc..... and vice versa.
Like that great song says.. "You gotta keep 'em seperated"
When i bought my africans the guy at my lfs said his tank contained both and ive seen africans with oscars on another forum although people there advised against it, i just stuck with africans but it can be done if you get the hardy species and if the individual fish has a nice personality
heres a link to another discussion from a different forum