Gene, tell me if I'm wrong, but I think what your wanting, is to have your 859 di transducer mounted next to your 998, on the transom and view both units from the console.If so, then just run the 998 on 455khz and the 859 on 800khz . shouldn't matter how close the transducers are to each other.the 2nd probably will have to be on different frequencies also