Common company ownership, different pyes-combine plans for testing?

[Guest TAshley]

I have a client that owns two companies. One company has a December year end and one has a September year end. I have been told that I must pick one of the year ends and combined the data for both of the companies as of that date for coverage and ADP/ACP testing. I was under the assumption that you could only combine plans with the same year end. Do the plans need to be combined for anything? Any guidance would be appreciated!