I have read a lot about people going to the hdsi transducer. I just saw a youtube video showing the mounting of both the compact transducer that they called high speed, and the hdsi transducer. They showed mounting the hdsi transducer more flush with the bottom of the boat, so it wouldn't throw a rooster tail on plane. My question is would it be to my advantage to run both? I understand that I would need to use a Y cable. Btw, I have an aluminum G3 deep v boat.
My unit is almost 2 years old, so trading with humminbird not really an option.

"high speed" is the transom dual beam transducer XNT 9 20 T. There is a non-temp sensor model XNT 9 20 that is used with another transducer and Y cable.

The idea of doing that with a metal hull is to put the more expensive SI transducer above the hull bottom for impact protection and sacrificing the cheaper dual beam transducer in case of impact. Also the smaller transducer may provide better high speed performance due to less water contact.
The Y cable is the AS SIDB Y.

The transducer that comes with the 899ci HD SI Combo is the XNT 9 SI 180 T and has the exact same mount as the XNT 9 20. There is a roostertail plug included with that mount.
The HDSI is constructed different and does not create a roostertail like the XNT mount does. It will still create some if not mounted correctly. Read the installation instructions.