The 859ci HD DI model unit should be released about that time.

No guarantees here that you won’t get interference but try setting your 998 unit to the 800kHz Si frequency. The new 859ci HD DI unit will need to have the Beam Select menu set to the 455kHz setting and the Imaging Frequency menu set to the 455kHz setting as well.

I don't know if you will have enough room on your trolling motor for both transducers.