They've been reported to, but it's more of a case of sticking a desperate waterbuck in with a single lechwe female and giving him plenty of tries. I hunted a 7,000 acre ranch with both species and they pretty much ignored each other and stayed by themselves.
However, waterbuck bulls have been known to turn down right vicious during the rut. Not only will they go out genocide against other bulls, they will kill the males of other species as well. I've heard of them killing sable..