Controlled Group Transition Rule Applicablity

I am getting conflicting answers as to whether the transition rule for acquisition solely applies to 410(b) or whether we can apply it to 401(a)(4) testing also.   

Situation is company B becomes a controlled group with Company A due to ownership change on 4/1/18.   Company A and Company B each sponsor 401k plans with calendar plan years.  A's is a cross tested Safe Harbor 401k, B's is a cross tested non-Safe Harbor 401k.

Do you think we  have to combine them for testing 401(a)(4) for the 2018 and 2019?  What about ADP testing since B is not a Safe Harbor 401k?

Does anyone know of a cite that addresses this?

The 410(b)(6)(C) transition rule by its terms applies only for 410(b), and there are no regulations under it (other than the helpful, but limited Treas. reg. 1.410(b)-2(f), which tells us that the form of the transaction, i.e. stock acquisition, purchase of assets, or merger does not matter). However, 410(b) is all you should need, right? As long as you do not change the eligibility for the two plans (e.g., they each continue to cover employees of the pre-transaction businesses, including new hires in the legacy businesses, at least within limits), you can test them separately for 2018 and 2019, even if they would not satisfy 410(b) otherwise on a separate basis. So for one you test with ADP/ACP, the other is a safe harbor. That should be fine.