Both FishDeeper app and Deeper Smart Sonar are fully compatible with any Deeper sonar devices. We advise to use the FishDeeper app, as it contains many improvements of connectivity and usability, and will perform much better with the Deeper Sonar.
FishDeeper and Deeper Smart Sonar apps can be installed at the same time one the single smartphone, and can be used interchangeably. All the data will be synced between the apps - you just need to go to the History section of the app that you have used, and click the cloud button to initiate the sync.