I received a response from Humminbird:

“Thank you for contacting Humminbird Technical Support and we appreciate your support of our products.

Unfortunately no, the unit only has one transducer port and there is not a y-cable or transducer switch that support the application where you connect both a trolling motor transducer and a transom transducer to one unit at the same time.”

I think for the time being, I’ll just unplug one & plug in the other when not in use.
Thanks for all the replies

It sounds like you all are suggesting the 5 port ethernet hub with another head unit. I see how that will work, and share info from different transducers through networking. However, I was trying to stay with only one head unit, Helix 10 Mega SI, and choose between the two transducers.