Bob B,
Same-same.
The Down Imaging comes from the right (Starboard) transducer in the AS-14-Si-LR-Y cable just as it does in the pre-ONIX version AS-Si-LR-Y cable. Only the left (Port) Si sonar comes from the port side transducer.
You can use a separate 2D transducer with the ONIX units as well and it works just like the pre-ONIX units in that the AS-14-SI-DB-Y splitter cable disconnects the 2D sonar from the ONIX HDSi transducer, so that the 2D sonar always comes from the 2D transducer.