This is the cable for hooking 2 xducers (1 SI and 1 dual beam) to 1 head unit...(without a switch)

AS SIDB Y
Accessories
720055-1
Side Imaging Splitter Cable
Y Cable - Side Imaging and Dual Beam - allows you to use two transducers on one unit without the use of a transducer switch. For use with Side Imaging products only, in conjunction with a dual beam transducer only. For improved high speed performance for bottom tracking.
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MSRP $29.99
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There is the option of the TS3 switch like cray mentioned to use 2 SI xducers ...it comes with 3 short cables.(1 to the head unit and 2 to the separate xducers)...

TS3 :: Humminbird :: Transducer Switch

Rickie