Typically the deeper in the water you can get a transducer on an aluminum boat the better it will work at faster boat speeds. So I would start with mounting the XNT-9-20-T transducer just to the left of the right side running board or maybe even a few inches closer to the motor than that.

The HDSi transducer will need to go on the opposite side of the motor and the farther away from the XNT-9-20-T transducer the better so that it will not block as much of the right side Si sonar signal. Be mindful that it still needs to be below the level of the motor mount though. Looks like you will have to trim the motor upwards when using the Si sonar of the 898c Si unit so that it will not block any of the right Si sonar. Hard to tell from the one photo where the best location would be.