Welfare Plan: Single vs Multiple Employer Plan

Company buys another smaller company and keeps its operations separate with its own EIN.

But all employees are in the owner company's medical and benefit plans. No specific joinder agreement was signed.

Two options of how to treat:

• Continue filing as Single ER
• Make them sign a joinder agreement and identify as multiple ER plan, with the understanding that prior year is being signed as Multiple ER based on currently signed Joinder agreement

Has the health plan’s or welfare plan’s administrator yet done an analysis of whether the two distinct companies or businesses are one employer within the meaning of Internal Revenue Code § 414(b)-(c)-(m)-(n)-(o)?