Top Heavy Calculation in Multiple Employer Plan

Company A and Company B constitute a brother-sister controlled group. Co. A sponsors a calendar-year end 401(k) plan, which Co. B has adopted. 

In June 2019, a business transaction occurs and Co. A and B are no longer a control group, but there remains shared level of ownership (just not enough to be considered a single employer plan). Employees of Co. A are "shifted" to Co. B; thus, in 2019 some employees begin to have account balances attributable to Co. A and Co. B. 

As this is a takeover plan, I don't have ALL the details for 2019 (such as the Top Heavy Test results as of 12/31/19). I do know the plan was not top heavy as of 12/31/2018.

I am working on the 12/31/2020 compliance testing and Form 5500. I am trying to back into the 12/31/19 top heavy account balances to determine if a top heavy minimum contribution is required. I know for a Multiple Employer Plan, Top Heavy, ADP,/ACP, coverage is calculated separately for each employer. I don't know how the testing was run in 2019, but my main concern is whether a 2020 TH minimum is required.

For participants who were employed by Co. A and B in 2019, how do I treat their account balances for Top Heavy determination? Initially, I intended to split out the accounts such that each affected participant has two accounts - one from Co A and one from Co B. as of June 2019 (and pro-rate earnings after June 2019). But, then I started thinking, since they were controlled for part of the year, can I split out the accounts as of 1/1/2020 for top heavy testing? I don't believe there is any formal guidance from IRS on this issue - its just any "reasonable" approach. 

Thoughts/opinions? Thank you!

There are those A employees who have not shifted over to B. There are those B employees who have always worked for B.

The problem, as I understand it, is the treatment of former A employees transferred to B ("Transferred Employees"). They are new employees of B as of June 19th, and any contribution allocated to their accounts post June 19 to December 31st, 2019 will be part of Plan B assets and tested as of December 31, 2019 for B's top-heavy status. Their account balances as of June 19th plus earnings thereon will be tested as of December 31, 2019, as a part of A's top heavy testing.

Are any of the Transferred Employees Key Employees of A or B during 2019?  

Yes, exactly, the treatment of the "Transferred Employees" is the issue.:) And yes, there are Key Employees who are included in the Transferred Employee group. 

It's my understanding the IRS hasn't issued clear guidance on how to treat mid-year transactions. From the Regs:

§1.401(k)-5 Special rules for mergers, acquisitions and similar events.

 [Reserved].

...and...

From IRS Notice 2000-3: POTENTIAL APPROACHES FOR APPLYING VARIOUS QUALIFICATION REQUIREMENTS IN MERGERS, ACQUISITIONS, DISPOSITIONS, AND SIMILAR TRANSACTIONS

Notice 2003-3 requested comments on proposed methods on how to approach testing, etc.. when business transactions occur mid-year.

I think there are a few reasonable approaches to this. Just make sure that there is a legitimate business reason for the transferring of the employees. If those underlying facts are sound, you should have flexibility on a reasonableness basis.