You cannot get DI from a SI transducer for the DI unit.
The only thing the 788ci HD DI Combo can share from the 1199ci HD SI Combo is 200 kHz 2D sonar.
The DI transducer is not shoot-thru advisable anymore. The XIH 9 DI T has been discontinued.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1