You have interference from the two sonar's seeing each other's return signals.
If you are operating the 778 in the quadbeam mode, your cone size is 2:1 (near 90deg) vs your depth, so if you're in 20' of water, you'll have a near 40' sized cone - I bet you don't have a 40' boat.
You can mitigate this down a bit by changing the frequencies to try and diminish the amount of cone overlap, playing with the noise rejection settings, and by setting the bottom to keep the unit from "guessing" where in the return the bottom really is, but due to the way that HB designed their system you'll always have a little bit of intereference unless you install some kind of bypass/transducer selector switch.
Or, you could just turn off the offending unit.
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