Varsity981,
Welcome to the forum. First, the Y cable will eliminate the 83/200kz 2d portion of the SI transdcuer completely and does not automatically switch between the two transducers after 10mph. If someone told you that, they are comepletely wrong! Thats why you are not getting any readings from the 2d of the SI transducer. The only way to use both for 2d would be to put in a transducer switch.
Then as for losing bottom at speed, how did the shoot-thru-hull transducer perform on your old unit at speed? You could hook it up directly without the Y cable (eliminate the SI for now) and see how it performs with this setup.
I would recommend updating your transdcuer. If I am not mistaken, the Wide128 only used 200khz, correct? So you also do not have any 83khz with this setup as well.
I hope that helps a little!
Robert