Hey ranger,
the y cable is for sharing 2 transducers with one unit. Connecting 2 units to the same transducer CANT go well, since both will start sending signals and waiting for the answers while reciving the other units echo too. I even believe that both, the units and the transducer will get damaged, they're running un high voltage.
I think the only way for using 1 transducer for 2 units is the interlink connection.
Greetings,
Del