Contribution Rules When Have More than one participating employer

Two dentists (husband and wife) have separate corporations.     Their corporations are both participating in  the same 401k plan, which has a safe harbor matching provision.  The husband and wife both make the maximum 401k contributions to the plan from their separate payrolls.    The wife's corporation contributes the safe harbor matching contribution to the plan for her, as well as the safe harbor contribution for the husband.   Is this OK, and does the wife's corporation get a tax deduction for both safe harbor contributions.

In general, in a plan with multiple participating employers, can one entity contribute for another entity's participants.

An employer can only contribute for its own employees and a contribution for an employee of another employer would violate the exclusive benefit rule.

To be tax deductible plan contributions must be an "ordinary and necessary" business expense of that taxpayer and a contribution for a non-employee can't meet the O&N standard.

However, in the H&W situation you posit I might document the contribution as a loan from W's corp to H's corp and take the deduction on the H corp tax return.