Try what Humminbird told you. Yes, you can mount 2 transducers and use both at the same time but you have to set each bird to run on different frequencies in their respective settings to not cross talk and show interference. You can network them to show waypoints, but not charts or mapping. If using one transducer, you could mount the 9 transducer and network the 7 to it and the 7 can be set up to read 2d, DI and SI. I don’t think the 9 will read DI off the 7’s transducer but would read 2d off it and it would have no SI as it has to read its own transducer for SI. Lots of ways to choose and play with network settings when both are networked together.