Stoporilshoot,
It would not be a problem if the transducer is mounted on the left side of the motor/prop. This statement was originally put into the install guides due to inboard motors/props. If the prop is behind the transducer it will not interfere with the sonar at all (though installing a transducer directly in front of the prop will cause problems…).
As for installing the transducer on the starboard side next to the other transducer: the other transducer would block some of the Si sonar to that side. This goes for installing the Si transducer on the port side with the other transducer on the starboard side as well but to a lesser degree. The other transducer will still block some of the Si sonar to the right side but not as much since it is farther away from the Si transducer. You can illustrate this for yourself by holding your arm out and sticking your thumb up. Sight along your arm and you see that your thumb does not block much of whatever it is you are looking at. Now bring your thumb closer to your face and you find that that same thumb now blocks more of your vision area – same goes for the other transducer being closer to or farther away from the Si transducer. The Si sonar beams are pretty narrow so you may be able to find a way to mount the Si transducer on some sort of block or standoff so that it is aft of the other transducer o that it does not block any of the Si sonar.