Greg is not a good example for what you asked. You asked about someone who has two employers and will be paid via payroll (and W2). Relationship matters: "Now switched employers"

It needs to made clear, when working with these types of benefit topics, that there are plans (employer established), and there are accounts (individual owned), and there are the people with a relationship as employer and/or employee. That and/or is important. The type of plan/account matters, as well, because, for instance, a SIMPLE IRA and a 401(k) stand separately; but, a 401(k) and a Solo 401(k) are both 401(k) and are not separated. A SIMPLE IRA and having a traditional or Roth IRA also are not separated, for instance, you see that on a 1099-R or Form 8606.

If your taxpayer that has 1099-NEC earned income and files Sched C and establishes a SEP-IRA (which is an account) or a Solo 401(k) (which is a plan, that provides for an account), there are different provisions. Under these conditions there is both an employer and an employee, even if that is the same person. But that means there are those separate provisions, depending on which hat your taxpayer is wearing at any point in the discussion.

Greg cannot make more contributions to the Solo 401(k) plan as the person earning the income, if Greg already maxed out employee contribution to a employer's 401(k) plan. However, Greg's Sched C "employer" (company) named Greg can still provide the employer share to the Solo 401(k). Watch the terminology:

"elective salary deferrals" means from the earned income the employee or the person "treated as employee" even though Greg as Sched C "owner/employee" doesn't earn a Salary.

"nonelective contribution" means from the employer company or the person "treated as the employer whose own company would be employing the person doing the work, if there was an employer/employee relationship provided for."

I found an article that might be more applicable:

https://www.cerebraltaxadvisors.com/blog/multiple-401k-accounts/

Hope that helps.