Not sure that I am understanding you here ccollins.
What transducer is on your trolling motor now?
You already have a 999ci HD Si unit mounted at the bow now, correct?
You are looking at getting the new 859ci HD DI model to install at the bow as well?
What transducer were you looking at using with the 859ci HD DI unit?

The Si and DI model units use the same frequencies, so depending on how you have them configured you may have sonar cross-talk problems.