First off I would see if I could send back the InterLink for an Ethernet Connection cable. The InterLink is more limited in what data can be shared or transferred across it versus the Ethernet connection. It also costs more.

You need to try and mount the transducers as far apart as you can so that the XNT-9-20-T transducer will be blocking less of the Si sonar. The farther away the better so that means on opposite sides of the motor. Hard to say where to mount the transducers as we cannot see the underside of the hull so we know where the strakes and rivets or other hull protrusions are at. It is best not to mount the transducers behind these. Maybe someone with the same boat will chip in and show where their transducer(s) are mounted.

When you use the AS-Si-DB-Y cable, the 2D sonar always comes from your XNT-9-20-T transducer. There is no automatic or manual kick in of one transducer versus the other.