if retirement benefits are the subject of good faith bargaining then that group is considered a separate plan for purposes of applying tests for coverage and nondiscrimination. you can continue under one plan (document, investment lineup, etc.) with different benefit structures but your document must be able to accommodate and be properly completed/amended. it may be simpler to set up a separate plan - and could be more expensive because now you have two sets of accounting, asset pools/investment expenses, 5500's etc., but there could also be savings if the plan had over 100 participants and needed an annual audit but then splits into two plans under 100 participants with no audit requirement.