Thanks for the replies. I thought the same thing about linking the two together but the helix 7 is not compatible with the 787, guess it's too old. I know a lot of bass fisherman use a second sonar up by the bow but have read mixed reviews on putting both pucks on the transom. Some seem to think if it runs on a different frequency it will work. I guess I'll try it out before drilling any holes and permanently attaching the transducer incase I need to move it to the bow. I think I'll shoot hummingbird an email and see what they recommend. I wanted to put it on last year but once the boat was in the water I didn't take it out until the season closed.
I run a sidescan transducer and two others, the only thing I had to worry about was that the sidescan was lower than the others. One of the others goes into the Lowrance with the sidescan, and the 3rd is a redundant on a Raymarine. No issues with either, run them together all the time.
I emailed hummingbird and they gave me a vague/indecisive answer... basically that it may or may not work, being they are both hummingbird products they operate on a similar frequency. They suggested to try switching the frequencies around or to shut the sonar off of one when using the other. They said if that don't work to try moving the ducer' to the bow.