Don,
If that is a 500 series unit that you have and it has the Si sonar, than it has to be the 598ci HD Si model.
As Robert said you could use a Unit Switch (US3) and change back and forth to connect one unit or the other (not both at once) to the same transducer. There is no way to network or share the sonar data to or from a 500 series unit like the 598ci HD Si. You would need at least a 798ci HD Si unit for this.
A Si transducer on each side of the transom should show a lot of interference in the Si sonar as both transducers will have a beam pointed directly at the other transducer. There may also be sonar interference in the 2D sonar.