If you have a stepped transom, you could do something similar to this:
I have a HDS 8 w/ SS mounted on my Tracker PT175, the SS ducer is under the step transom close to centerline of the boat and the 2d ducer is mounted 12" +/- to the right when looking from the stern forward. My SS ducer is about 1-2" or so lower than the 2d, no issues.