Are you talking about the AS-Si-DB-Y cable? If so, that is to connect a 2d transducer with a SI transducer for on plane readings. If you are referring to the AS-SILR-Y cable, that is for a dual SI setup, where something (usually a sterndrive) is blocking the SI signal, so one is needed on each side of the stern. You get an image from each transducer with this method.
From your description, you want the TS3 switch. And you should be able to find it for less than $85
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