Welcome to the Humminbird Side Imaging Forums Ryans Pappa.
No, you do not have to connect both units together, you can keep them separate.
How do you have your trolling motor connected to your 998 unit?
Is this the sonar only or is this an i-Pilot Link trolling motor?
If this is an i-Pilot Link, then yes you would need the 5-Port Ethernet Switch and extra cables to connect everything together.