Issues using Two Transducers on Humminbird Fish Finder

[MikeG3Boat]

I am using two transducers on my Humminbird 1198 and I am having an issue with my 2d sonar picture at higher speeds (>20).   They are connected using a Y-cable from humminbird.  I was working on making adjustments to my side-imaging transducer for this to work better, but at this time I have not been able to make any real improvements. I called up customer service on another question and they informed me that my 2d sonar signal is coming from the non-side imaging transducer.  That seems odd to me that it wouldn't be getting all of its signals from one transducer or the other transducer.  Before I go messing around with the placement of my other transducer I wanted to ask everyone here if they know this to be true? 

I have already checked many of the initial things that can cause issues with sonar readings, like line of rivets, etc.  Any help would be appreciated.  Thanks!

[h8go4s]

http://www.humminbird.com/Category/Support/FAQ/SideImagingSplitterCables/

[AnderJA]

You si and di come from the side transducer and you 2d sonar and depth comes from the 2d sonar. the 2d sonar is the one you need to be adjusting to get better on plane readings. If you have glass boat then you can do a thru hull transducer and that seems to work best but it's not possible for aluminum boats.