Do you see any issues with the following structure satisfying the 401(k) safe harbor rules.

Plan X provides for two safe harbor formuals. Employees in Group A get a the NEC. Employees in Group B get the basic match.

At the very least the match and nonelective would have to pass coverage testing.

In other words, you can have separate formulas (assuming other requirements met), but you treat the parts of the plan as disaggregated under the coverage rules. Right?

You mandatorily disaggregate for coverage per 1.410(b)-7(c). All statutorily includible NHCEs must be covered under one of the safe harbor formulas.

I have not been able to find objections to your proposed arrangement. If there are any problems with it, the brilliant minds on this forum will identify them.

If you are trying to satisfy the 401(k) / 401(m) safe harbor plan design conditions, this doesn't work. The entire plan must satisfy one of the safe harbor plan designs for the entire plan year.