I have a pair of 587 cihddi humminbirds. If I go into my menu and select 455khz on my 2d sonar I loose signal. If I am on the split 200/455 2d screen the 455 is blank and 200 is normal.

This is the same on both units. They are on separate ducers and not connected in any way.
Ist there a menu option I am missing or do I have a problem?