we know if you have a controlled group, you have to include all employees for coverage purposes.
if you aggregate for coverage, then you have to aggregate for nondiscrim (ADP testing) as well.
but you can't do that unless both plans use the same testing method - but there is no requirement that all members be safe harbor.
Otherwise the regulation (1.401(k)-1(b)(4)(iii)(B)) that says you may not aggregate a plan using ADP safe harbor provisions with another plan that is using the ADP test makes no sense. (This particular section of the regs will refer you to 1.410(b)-7(d) which are the rules for permissive aggregation (refers you to the 'employer' choosing how to treat 2 or more plans, including QSLOBs, etc)
Under what other circumstances would you have the situation arise where you would have a non safe harbor 401(k) and a safe harbor 401(k)? you can't (Tom corrected his typo) aggregate a plan with a non-related company.
I suppose you could have an odd company that sponsors both a 401(k) plan and a safe harbor 401(k) plan...
possibly what you might have read or are thinking of, is that even if both plans are safe harbor, they have to have the same formula as well.