i don't know of any congo buffalo/bongo mixed species exhibits at any zoos, but i so do think it would be nice. i love those congo documentaries and photographs where you see herds of the two species grazing together in jungle clearings.
I've often wonder why several of the Congo species are orange or red coloured- Congo buffalo, Bongo, Red River Hog etc. Is it to match the soil? Bongo are increasingly common in UK zoos nowadays, even a number of Safari Parks have them too. Congo Buffalo are less common over here but a few zoos have them (I think they're far more attractive than the Big buffalo species). No-one over here has tried a mixed exhibit yet. I guess one day in the future somewhere with a lot of money will attempt a massive 'Congo' complex to house these species, plus perhaps Gorillas and other species mixed in too?