jtd123,
Welcome to the forum. Yes, the flat one on the transom is for Side Imaging. If there is a switch, you will have SI and depth (2d) from the transom and will be able to switch to the in-hull for better 2d depth reading at high speeds.
Most likely, you have an SIDB Y cable installed that will take the SI from the transom transducer and the 2d depth from the in-hull. This is the prefered method so you dont have to worry about switching between the two.
You will be able to show them side by side with Y cable.
What else can we help with to get you started?
Robert