First you would need a Helix 9, Helix 10, or Helix 12 of any version.
Then each would need an Ethernet adapter --- AS EC QDE.
Connect the two adapters with an Ethernet cable --- AS EC 20E is the 20' version.
From there you can not only share a transducer, but also sonar technologies that one has and the other doesn't (Cross sharing). You can also share GPS data and sensors plus any other Ethernet accessory like RADAR and 360.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1