Solo plan plus a 401k plan - necessary?

A medical professional worked for a hospital but had non-hospital income so she set herself up as a sole prop and started a solo plan for herself with the outside income used as a basis for contributions in the solo plan.

She has since left the hospital and started her own small practice.  She wants to have a 401k plan for the practice while keeping her solo going with the ongoing outside income.

Even though the income for the solo is different source than that for the new practice, these would be related businesses, am I correct?  She was doing after-tax in the solo.  Assuming the few employees in the new practice would not, then her after-tax would now be zero?

Thank you

She owns 100% of her Sole Prop, and 80% or more (I assume) of her own "small practice." This is a controlled group under Section 414(m). Don't confuse the 414 rules, which tell you "who is the employer" with the "same trade of business" rules which tell you whether different streams of earned income may be combined for purposes of determining "plan compensation."

Yes, control group.

Yes, no advantage to having separate 401(k) and VAT plans, this can be one plan (but...)

Yes, likely no future VAT because of ACP testing.

If each entity is a participating employer, then her pay from each can be compensation.

If for some other weird reason she wants separate plans, be wary of BRFs issues if her "SE" plan has features not in her practice 401(k).