Both units use the same frequencies. The 1199 uses 200 kHz all the time regardless of what 2D frequency you choose to view.
Like frequencies will cross-talk when the pulses overlap. It is a sure thing when they are that close.
Set the L to use 83 kHz and use different SI and DS frequencies.
The SI transducer has to be low enough so the SI pulse can "see" past the hull sponson.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1