Suppose an employer sponsors a safe harbor unit benefit DB. If the same employer adopts a safe harbor 401(k) are we required to general test accross two plans?

No. First, if you have 2 safe harbors you don't have to general test at all.
Second, if one were not a safe harbor and you needed to general test it, you would not necessarily need to include the other. That would depend upon the specifics of how you were testing.

Thanks Andy