“AS-Si-LR-Y is right, so I guess there would be no physical interference with the xnt 9 between them.”
No there would not be. Each HDSi transducer will be shooting the Si beam that is connected towards the outside of the boat away from the DI transducer.
“But since 1198Si is Di split, is the Di from the 688 going to be an issue running at the same time?”
Yes if they are running at the same frequency.
“And is the 2d running on the Si's running when not on the screen? (I'm guessing yes)”
Yes, the 200kHz 2D sonar on the Si units is always running as long as the sonar is on.
“But if only running ilink/iPilot, charts, recording and Si on the 1198si going to cause 2d interference?”
Yes if you are using the 200kHz 2D sonar in the DI unit.
“I plan to use the 688 HD Di for a dedicated Di & 2d at times. Looks like I can turn 2d off on 688 but can I on 1198?”
No, the 200kHz 2D sonar on the Si units is always running as long as the sonar is on.