Plan Merger and Prior Year Testing

Company A:  Plan uses prior year testing method.  

Company A acquires company B on 9-1-21. 

Company B uses current year testing method

Effective 12-1-22, Company/Plan B merges into Company/Plan A.  We are testing Plan A for full year and including Plan B only from 12/1-12/31.  

Both populations are approximately the same.  What average do I use for the prior year testing adp/acp average?  Do I need to do a weighted average?  (Ideally, they should have waited to merge until 1/1/23. Or tested together for entire plan year).

Thoughts?