Many Side Imaging unit owners use a second transducer for 2D sonar readings for one of two reasons:
1. The Side Imaging transducer they have cannot be mounted in a location on the boat that allows for unblocked Si sonar and high-speed water depth readings. They also may not want to deal with having to possibly move the transducer multiple times (more holes in the boat) to find that mounting location.
2. They want to mount their Side Imaging transducers in a more protected location on the boat so that it does not get damaged from winterbourne debris, rocks or stumps and logs.
Yes, the XNT-9-Si-180-T Compact Side imaging (CSi) transducer can be used for high-speed water depth readings but it is usually easier to mount a smaller sized transducer on the transom of a boat (or one that can be mounted inside-the-hull) than the larger CSi transducer. This is even truer with the even larger HDSi transducers.