Coverage Testing Multiple Plans When Employees Transfer

Three employers are members of a controlled group (A, B, C) and they each sponsor a 401(k) plan with different features.  Each plan passes coverage separately - but barely.

Let's say an employee transfers mid-year from Employer/Plan A to Employer/Plan C, is that employee considered in Plan A only or Plan C only or both plans for coverage purposes?

Thank you for any insights and references!

If an employee is eligible to make a deferral in a plan they are considering benefiting under that plan for purposes of 410(b), so in your case it appears to be both plans.