Must 401(k) Safe Harbors be made for all plans of an employer?

[ERISA1]

I've got an employer that sponsors two separate 401(k) plans. Each plan satisfies 410(b) and all other tests separately (i.e., aggregation is not required). The employer wants to make a 3% non-elective safe harbor election with respect to one, but not the other, plan. Employees participate in one plan or the other; no one participates in more than one plan.

I know you couldn't make catch-up contributions available in one plan only, but it seems to me you can limit a safe harbor to the participants who are eligible in just one of the plans.

Along the same lines, do you think that one plan could be cross tested and the other not?

Any thoughts?