No, I think you've got things a bit mixed up.
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I read in the ERISA outline book that if you have a plan that covers union employees, it is deemed to pass coverage.
No, if the plan ONLY covers union employees (and meets other criteria-bargained in good faith, etc). You can't get a free pass by including one union person. Only part of that plan would get a free pass.
If you have one plan that is non-union and another that has both, you have three plans for testing purposes. The "union" group is treated as if they are in a separate plan.
See 1.410(b)-(6)(d).
And be careful of the word union. Collectively bargained would be more accurate. There sometimes are employees who are subject to a collective bargaining agreement that are not "union". And not all union people meet the collectively bargained exemption.
Hope this helps. I think the cite will explain the testing rules.