2 related employers maintain their own calendar year 401k plans and the plans aren't aggregated for testing. Both don't have matching contributions. One employer has a safe harbor 3% and the other employer's plan is a nonsafe harbor plan subject to normal ADP testing. The employers now wish to merge the nonsafe harbor plan into the safe harbor plan before the end of the year.

Can this be done? We assume the merger will allow short plan year ADP testing through date of merger for the nonsafe harbor plan. What are the issues?

I think that's how I'd do it (short year test in the non-SH plan and then SH contributions from the point of merger). I'm not aware of any official guidance.